NFC: digital: Randomize poll cycles

This change adds some entropy to polling cycles, choosing the next
polling rf technology randomly. This reflects the change done in the
pn533 driver, avoiding possible infinite loop for devices that export 2
targets on 2 different modulations. If the first target is not
readable, we will stay in an error loop for ever.

Signed-off-by: Thierry Escande <thierry.escande@linux.intel.com>
Signed-off-by: Samuel Ortiz <sameo@linux.intel.com>