NFC: trf7970a: Rename TRF7970A_ST_OFF to TRF7970A_ST_RF_OFF

Rename the 'TRF7970A_ST_OFF' state to 'TRF7970A_ST_RF_OFF'
to make it clear that this state means that the RF is
off and not the entire device.

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