net: fec: ptp: fix NULL pointer dereference if ptp_clock is not set

Since commit 278d2404 (net: fec: ptp: Enable PPS output based on ptp clock)
fec_enet_interrupt calls fec_ptp_check_pps_event unconditionally, which calls
into ptp_clock_event. If fep->ptp_clock is NULL, ptp_clock_event tries to
dereference the NULL pointer.
Since on i.MX53 fep->bufdesc_ex is not set, fec_ptp_init is never called,
and fep->ptp_clock is NULL, which reliably causes a kernel panic.

This patch adds a check for fep->ptp_clock == NULL in fec_enet_interrupt.

Signed-off-by: Philipp Zabel <p.zabel@pengutronix.de>
Signed-off-by: David S. Miller <davem@davemloft.net>