phy: mvebu-cp110-comphy: Cosmetic change in a helper

Before adding more logic, simplify a bit the writing of the
mvebu_comphy_get_mode() helper by using a pointer instead of
referencing a configuration with the entire table name.

Signed-off-by: Miquel Raynal <miquel.raynal@bootlin.com>
Signed-off-by: Kishon Vijay Abraham I <kishon@ti.com>