ksz8021_config_init() unconditionally sets the KSZPHY_OMSO_RMII_OVERRIDE
bit. This is since the initial micrel phy commit, so it's not
reproducible where this comes from and why this is done. Neither U-Boot
nor the kernel ever touch this bit and so should we. Also, instead
of doing a write only operation, read/modify/write the bit we actually
want to change.
This fixes operation on a KSZ8081MLX which is a MII only phy.
KSZPHY_OMSO_RMII_OVERRIDE is reserved here and must be written to 0.
KSZPHY_OMSO_MII_OVERRIDE is default 1 and must be written as 1.

Signed-off-by: Sascha Hauer <s.hauer@pengutronix.de>