phy: Return NULL if the phy is optional

If we're trying to get a handle to an optional phy, then the phy framework
being disabled shouldn't return an hard error.

Instead, return NULL just like phy_optional_get does when there's no phy
provided in the DT.

Signed-off-by: Maxime Ripard <maxime.ripard@free-electrons.com>
Signed-off-by: Kishon Vijay Abraham I <kishon@ti.com>