net: phy: possible NULL dereference in fixed_phy_create()

We check if phydev is NULL. Only but if it is non-NULL we set one
component of phydev. But even if it is NULL we set another. We should not
dereference NULL in either case.

Fixes: e24b58f5 ("net: phy: don't require PHY interface mode during PHY creation")
Signed-off-by: Heinrich Schuchardt <heinrich.schuchardt@canonical.com>
Reviewed-by: Marek Behún <kabel@kernel.org>