net: fec: introduce fec_ptp_stop and use in probe fail path

This function frees resources and cancels delayed work item that
have been initialized in fec_ptp_init().

Use this to do proper error handling if something goes wrong in
probe function after fec_ptp_init has been called.

Signed-off-by: Lucas Stach <l.stach@pengutronix.de>
Acked-by: Fugang Duan <B38611@freescale.com>
Signed-off-by: David S. Miller <davem@davemloft.net>