phy: marvell: a3700: Use reg_set16 instead of phy_write16

The macro phy_write16 is not used by the rest of the code,
phy_read16 is not used at all.
We also change the macro SGMIIPHY_ADDR to a static inline function.

Signed-off-by: Marek Behun <marek.behun@nic.cz>
Signed-off-by: Stefan Roese <sr@denx.de>