NFC: digital: Add NFC-DEP Target-side NACK Support

NFC: digital: Add NFC-DEP Target-side NACK Support

When an NFC-DEP Target receives a NACK PDU with
a PNI equal to 1 less than the current PNI, it
is supposed to re-send the last PDU.  This is
implied in section 14.12.5.4 of the NFC Digital
Protocol Spec.

The digital layer's NFC-DEP code doesn't implement
Target-side NACK handing so add it.  The last PDU
that was sent is saved in the 'nfc_digital_dev'
structure's 'saved_skb' member.  The skb will have
an additional reference taken to ensure that the skb
isn't freed when the driver performs a kfree_skb()
on the skb.  The length of the skb/PDU is also saved
so the length can be restored when re-sending the PDU
in the skb (the driver will perform an skb_pull() so
an skb_push() needs to be done to restore the skb's
data pointer/length).

Reviewed-by: Thierry Escande <thierry.escande@linux.intel.com>
Tested-by: Thierry Escande <thierry.escande@linux.intel.com>
Signed-off-by: Mark A. Greer <mgreer@animalcreek.com>
Signed-off-by: Samuel Ortiz <sameo@linux.intel.com>