net: phy: fixed_phy: Fix NULL vs IS_ERR() checking in __fixed_phy_register

The fixed_phy_get_gpiod function() returns NULL, it doesn't return error
pointers, using NULL checking to fix this.i

Fixes: 5468e82f ("net: phy: fixed-phy: Drop GPIO from fixed_phy_add()")
Signed-off-by: Miaoqian Lin <linmq006@gmail.com>
Link: https://lore.kernel.org/r/20211224021500.10362-1-linmq006@gmail.com


Signed-off-by: Jakub Kicinski <kuba@kernel.org>