stmmac: intel: unlock on error path in intel_crosststamp()

We recently added some new locking to this function but one error path
was overlooked.  We need to drop the lock before returning.

Fixes: f4da5652 ("net: stmmac: Add support for external trigger timestamping")
Signed-off-by: Dan Carpenter <dan.carpenter@oracle.com>
Reviewed-by: Wong Vee Khee <vee.khee.wong@linux.intel.com>
Signed-off-by: David S. Miller <davem@davemloft.net>