net: phy: realtek: make define more consistent

All internal defines in the realtek phy are with a small X,
except MIIM_RTL8211X_CTRL1000T_MASTER. Make this more consistent

Signed-off-by: Olliver Schinagl <oliver@schinagl.nl>
Acked-by: Joe Hershberger <joe.hershberger@ni.com>