net: cavium: fix NULL pointer dereference in cavium_ptp_put

Prevent a kernel panic on reboot if ptp_clock is NULL by checking
the ptp pointer before using it.

Signed-off-by: Jan Glauber <jglauber@cavium.com>
Fixes: 8c56df37

 ("net: add support for Cavium PTP coprocessor")
Cc: Radoslaw Biernacki <rad@semihalf.com>
Cc: Aleksey Makarov <aleksey.makarov@cavium.com>
Signed-off-by: David S. Miller <davem@davemloft.net>