NFC: trf7970a: Don't de-assert EN2 unless it was asserted

When the trf7970a part has the bug related to 'en2-rf-quirk',
the GPIO connected to the EN2 pin will not be asserted by the
driver when powering up so it shouldn't be de-asserted when
powering down.

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