bnx2x: prevent crash when accessing PTP with interface down

It is possible to crash the kernel by accessing a PTP device while its
associated bnx2x interface is down. Before the interface is brought up,
the timecounter is not initialized, so accessing it results in NULL
dereference.

Fix it by checking if the interface is up.

Use -ENETDOWN as the error code when the interface is down.
 -EFAULT in bnx2x_ptp_adjfreq() did not seem right.

Tested using phc_ctl get/set/adj/freq commands.

Signed-off-by: Michal Schmidt <mschmidt@redhat.com>
Signed-off-by: David S. Miller <davem@davemloft.net>