NFC: trf7970a: Create startup and shutdown routines

Encapsulate the code to start up and gracefully
shutdown the driver and trf7970a device.  Doing
this makes adding system suspend/resume support
easier and the resulting code cleaner.

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