NFC: Core must test the device polling state inside the device lock

There can ever be only one call to nfc_targets_found() after polling
has been engaged. This could be from a target discovered event from
the driver, or from an error handler to notify poll will never complete.

Signed-off-by: Eric Lapuyade <eric.lapuyade@intel.com>
Signed-off-by: Samuel Ortiz <sameo@linux.intel.com>