net: stmmac: xgmac: use const char arrays for string constants

Jiri Slaby advises me that the preferred mechanism for declaring
string constants is static char arrays, so use that here.

This mostly reverts
commit 1692b977 ("net: stmmac: xgmac: use #define for string constants")

That commit was a fix for
commit 46eba193 ("net: stmmac: xgmac: fix handling of DPP safety error for DMA channels").
The fix being replacing const char * with #defines in order to address
compilation failures observed on GCC 6 through 10.

Compile tested only.
No functional change intended.
Suggested-by: Jiri Slaby <jirislaby@kernel.org>
Link: https://lore.kernel.org/netdev/485dbc5a-a04b-40c2-9481-955eaa5ce2e2@kernel.org/Signed-off-by: Simon Horman <horms@kernel.org>
Signed-off-by: David S. Miller <davem@davemloft.net>

drivers/net/ethernet/stmicro/stmmac/dwxgmac2_core.c

@@ -846,42 +846,41 @@ static const struct dwxgmac3_error_desc dwxgmac3_dma_errors[32]= {
 	{ false, "UNKNOWN", "Unknown Error" }, /* 31 */
 };
 
-#define DPP_RX_ERR "Read Rx Descriptor Parity checker Error"
-#define DPP_TX_ERR "Read Tx Descriptor Parity checker Error"
-
+static const char dpp_rx_err[] = "Read Rx Descriptor Parity checker Error";
+static const char dpp_tx_err[] = "Read Tx Descriptor Parity checker Error";
 static const struct dwxgmac3_error_desc dwxgmac3_dma_dpp_errors[32] = {
-	{ true, "TDPES0", DPP_TX_ERR },
-	{ true, "TDPES1", DPP_TX_ERR },
-	{ true, "TDPES2", DPP_TX_ERR },
-	{ true, "TDPES3", DPP_TX_ERR },
-	{ true, "TDPES4", DPP_TX_ERR },
-	{ true, "TDPES5", DPP_TX_ERR },
-	{ true, "TDPES6", DPP_TX_ERR },
-	{ true, "TDPES7", DPP_TX_ERR },
-	{ true, "TDPES8", DPP_TX_ERR },
-	{ true, "TDPES9", DPP_TX_ERR },
-	{ true, "TDPES10", DPP_TX_ERR },
-	{ true, "TDPES11", DPP_TX_ERR },
-	{ true, "TDPES12", DPP_TX_ERR },
-	{ true, "TDPES13", DPP_TX_ERR },
-	{ true, "TDPES14", DPP_TX_ERR },
-	{ true, "TDPES15", DPP_TX_ERR },
-	{ true, "RDPES0", DPP_RX_ERR },
-	{ true, "RDPES1", DPP_RX_ERR },
-	{ true, "RDPES2", DPP_RX_ERR },
-	{ true, "RDPES3", DPP_RX_ERR },
-	{ true, "RDPES4", DPP_RX_ERR },
-	{ true, "RDPES5", DPP_RX_ERR },
-	{ true, "RDPES6", DPP_RX_ERR },
-	{ true, "RDPES7", DPP_RX_ERR },
-	{ true, "RDPES8", DPP_RX_ERR },
-	{ true, "RDPES9", DPP_RX_ERR },
-	{ true, "RDPES10", DPP_RX_ERR },
-	{ true, "RDPES11", DPP_RX_ERR },
-	{ true, "RDPES12", DPP_RX_ERR },
-	{ true, "RDPES13", DPP_RX_ERR },
-	{ true, "RDPES14", DPP_RX_ERR },
-	{ true, "RDPES15", DPP_RX_ERR },
+	{ true, "TDPES0", dpp_tx_err },
+	{ true, "TDPES1", dpp_tx_err },
+	{ true, "TDPES2", dpp_tx_err },
+	{ true, "TDPES3", dpp_tx_err },
+	{ true, "TDPES4", dpp_tx_err },
+	{ true, "TDPES5", dpp_tx_err },
+	{ true, "TDPES6", dpp_tx_err },
+	{ true, "TDPES7", dpp_tx_err },
+	{ true, "TDPES8", dpp_tx_err },
+	{ true, "TDPES9", dpp_tx_err },
+	{ true, "TDPES10", dpp_tx_err },
+	{ true, "TDPES11", dpp_tx_err },
+	{ true, "TDPES12", dpp_tx_err },
+	{ true, "TDPES13", dpp_tx_err },
+	{ true, "TDPES14", dpp_tx_err },
+	{ true, "TDPES15", dpp_tx_err },
+	{ true, "RDPES0", dpp_rx_err },
+	{ true, "RDPES1", dpp_rx_err },
+	{ true, "RDPES2", dpp_rx_err },
+	{ true, "RDPES3", dpp_rx_err },
+	{ true, "RDPES4", dpp_rx_err },
+	{ true, "RDPES5", dpp_rx_err },
+	{ true, "RDPES6", dpp_rx_err },
+	{ true, "RDPES7", dpp_rx_err },
+	{ true, "RDPES8", dpp_rx_err },
+	{ true, "RDPES9", dpp_rx_err },
+	{ true, "RDPES10", dpp_rx_err },
+	{ true, "RDPES11", dpp_rx_err },
+	{ true, "RDPES12", dpp_rx_err },
+	{ true, "RDPES13", dpp_rx_err },
+	{ true, "RDPES14", dpp_rx_err },
+	{ true, "RDPES15", dpp_rx_err },
 };
 
 static void dwxgmac3_handle_dma_err(struct net_device *ndev,