net: stmmac: dwmac-meson8b: use FIELD_PREP instead of open-coding it

Use FIELD_PREP() to shift a value to the correct offset based on a
bitmask instead of open-coding the logic.
No functional changes.
Reviewed-by: Andrew Lunn <andrew@lunn.ch>
Signed-off-by: Martin Blumenstingl <martin.blumenstingl@googlemail.com>
Signed-off-by: David S. Miller <davem@davemloft.net>

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

@@ -5,6 +5,7 @@
  * Copyright (C) 2016 Martin Blumenstingl <martin.blumenstingl@googlemail.com>
  */
 
+#include <linux/bitfield.h>
 #include <linux/clk.h>
 #include <linux/clk-provider.h>
 #include <linux/device.h>
@@ -32,7 +33,6 @@
 #define PRG_ETH0_CLK_M250_SEL_SHIFT	4
 #define PRG_ETH0_CLK_M250_SEL_MASK	GENMASK(4, 4)
 
-#define PRG_ETH0_TXDLY_SHIFT		5
 #define PRG_ETH0_TXDLY_MASK		GENMASK(6, 5)
 
 /* divider for the result of m250_sel */
@@ -262,7 +262,8 @@ static int meson8b_init_prg_eth(struct meson8b_dwmac *dwmac)
 					PRG_ETH0_INVERTED_RMII_CLK, 0);
 
 		meson8b_dwmac_mask_bits(dwmac, PRG_ETH0, PRG_ETH0_TXDLY_MASK,
-					tx_dly_val << PRG_ETH0_TXDLY_SHIFT);
+					FIELD_PREP(PRG_ETH0_TXDLY_MASK,
+						   tx_dly_val));
 
 		/* Configure the 125MHz RGMII TX clock, the IP block changes
 		 * the output automatically (= without us having to configure