net: phy: xilinx: Check interface type in ->config(), not ->probe()

We want to be able to have phydev->interface uninitialized during
->probe(). We should assume that phydev->interface is initialized only
before ->config().

Signed-off-by: Marek Behún <marek.behun@nic.cz>
Reviewed-by: Ramon Fried <rfried.dev@gmail.com>