net: phy: marvell: Refactor m88e1121 RGMII delay configuration

Turns out that MII_M1116R_CONTROL_REG_MAC is the same as
MII_88E1121_PHY_MSCR_REG. Refactor the code to set the RGMII delays
into a shared helper.

Signed-off-by: Andrew Lunn <andrew@lunn.ch>
Signed-off-by: David S. Miller <davem@davemloft.net>