NFC: trf7970a: Use spi_sync() instead of spi_write_then_read()

To more efficiently handle long continuous reads,
use spi_sync() instead of spi_write_then_read().

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