net: stmmac: dwmac-meson8b: fix mask definition of the m250_sel mux

The m250_sel mux clock uses bit 4 in the PRG_ETH0 register. Fix this by
shifting the PRG_ETH0_CLK_M250_SEL_MASK accordingly as the "mask" in
struct clk_mux expects the mask relative to the "shift" field in the
same struct.

While here, get rid of the PRG_ETH0_CLK_M250_SEL_SHIFT macro and use
__ffs() to determine it from the existing PRG_ETH0_CLK_M250_SEL_MASK
macro.

Fixes: 566e8251 ("net: stmmac: add a glue driver for the Amlogic Meson 8b / GXBB DWMAC")
Signed-off-by: Martin Blumenstingl <martin.blumenstingl@googlemail.com>
Reviewed-by: Jerome Brunet <jbrunet@baylibre.com>
Link: https://lore.kernel.org/r/20201205213207.519341-1-martin.blumenstingl@googlemail.comSigned-off-by: Jakub Kicinski <kuba@kernel.org>

drivers/net/ethernet/stmicro/stmmac/dwmac-meson8b.c

@@ -30,7 +30,6 @@
 #define PRG_ETH0_EXT_RMII_MODE		4
 
 /* mux to choose between fclk_div2 (bit unset) and mpll2 (bit set) */
-#define PRG_ETH0_CLK_M250_SEL_SHIFT	4
 #define PRG_ETH0_CLK_M250_SEL_MASK	GENMASK(4, 4)
 
 /* TX clock delay in ns = "8ns / 4 * tx_dly_val" (where 8ns are exactly one
@@ -155,8 +154,9 @@ static int meson8b_init_rgmii_tx_clk(struct meson8b_dwmac *dwmac)
 		return -ENOMEM;
 
 	clk_configs->m250_mux.reg = dwmac->regs + PRG_ETH0;
-	clk_configs->m250_mux.shift = PRG_ETH0_CLK_M250_SEL_SHIFT;
-	clk_configs->m250_mux.mask = PRG_ETH0_CLK_M250_SEL_MASK;
+	clk_configs->m250_mux.shift = __ffs(PRG_ETH0_CLK_M250_SEL_MASK);
+	clk_configs->m250_mux.mask = PRG_ETH0_CLK_M250_SEL_MASK >>
+				     clk_configs->m250_mux.shift;
 	clk = meson8b_dwmac_register_clk(dwmac, "m250_sel", mux_parents,
 					 ARRAY_SIZE(mux_parents), &clk_mux_ops,
 					 &clk_configs->m250_mux.hw);