net: fec: defer probe if regulator is not ready

Defer probe if regulator is not ready. E.g. some regulator is fixed
regulator controlled by i2c expander gpio, the i2c device may be probed
after the driver, then it should handle the case of defer probe error.

Signed-off-by: Fugang Duan <fugang.duan@nxp.com>
Signed-off-by: David S. Miller <davem@davemloft.net>