NFC: trf7970a: Correct register settings for 27MHz clock

In prior commits the selected clock frequency does not propagate
correctly to what is written to the TRF7970A_MODULATOR_SYS_CLK_CTRL
register.

Signed-off-by: Geoff Lansberry <geoff@kuvee.com>
Acked-by: Mark Greer <mgreer@animalcreek.com>
Signed-off-by: Samuel Ortiz <sameo@linux.intel.com>