NFC: trf7970a: Only abort a command if one is active

Only initiate the abort command process when there is an active command.
If the abort process were started and there wasn't an active command
then the next command issued by the digital layer would be incorrectly
aborted.

Signed-off-by: Mark A. Greer <mgreer@animalcreek.com>
Signed-off-by: Samuel Ortiz <sameo@linux.intel.com>