phy: ti: gmii-sel: do not fail in case of gmii

The "gmii" PHY interface mode is supported on TI AM335x/437x/5xx SoCs, so
don't fail if it's selected.

Signed-off-by: Grygorii Strashko <grygorii.strashko@ti.com>
Signed-off-by: Kishon Vijay Abraham I <kishon@ti.com>