NFC: trf7970a: Add NULL check to clear up smatch warning

Although it should be unnecessary, add a NULL pointer check
to trf7970a_send_upstream() to eliminate a smatch warning.

Reported-by: Dan Carpenter <dan.carpenter@oracle.com>
Signed-off-by: Mark Greer <mgreer@animalcreek.com>
Signed-off-by: Samuel Ortiz <sameo@linux.intel.com>