phy: ocelot-serdes: convert to use eth phy mode and submode

Convert ocelot-serdes PHY driver to use recently introduced
PHY_MODE_ETHERNET and phy_set_mode_ext().

Cc: Quentin Schulz <quentin.schulz@bootlin.com>
Signed-off-by: Grygorii Strashko <grygorii.strashko@ti.com>
Reviewed-by: Quentin Schulz <quentin.schulz@bootlin.com>
Tested-by: Quentin Schulz <quentin.schulz@bootlin.com>
Signed-off-by: Kishon Vijay Abraham I <kishon@ti.com>