nfc: NULL vs zero in nci_activate_target()

This is a pointer so it should be NULL instead of zero.  Sparse
complains about this stuff:
net/nfc/nci/core.c:447:37: warning: Using plain integer as NULL pointer

Signed-off-by: Dan Carpenter <dan.carpenter@oracle.com>
Acked-by: Samuel Ortiz <sameo@linux.intel.com>
Signed-off-by: John W. Linville <linville@tuxdriver.com>