NFC: pn533: reset poll modulation list before calling targets_found

We need to reset the poll modulation list before calling
nfc_targets_found because otherwise userspace could run
before the modulation list is cleared and then get a "Cannot
activate target while polling" error upon calling activate_target.

Signed-off-by: Michael Thalmeier <michael.thalmeier@hale.at>
Signed-off-by: Samuel Ortiz <sameo@linux.intel.com>