NFC: trf7970a: Don't turn on RF if there is already an RF field

Currently, the trf7970a driver blindly turns on its
RF field when configuring its framing.  This isn't
a good idea if there is already a device generating
an RF field.  Instead, check if there is already an
RF field present before turning on this device's RF
field and, if there is, return EBUSY.

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