NFC: Set rf_mode to NFC_RF_NONE where necessary

rf_mode is now set to NFC_RF_NONE when a device gets allocated,
when the link goes down, and when stop polling.

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