NFC: digital: Fix ACK & NACK PDUs handling in target mode

When the target receives a NACK PDU, it re-sends the last sent PDU.

ACK PDUs are received by the target as a reply from the initiator to
chained I-PDUs. There are 3 cases to handle:
- If the target has previously received 1 or more ATN PDUs and the PNI
  in the ACK PDU is equal to the target PNI - 1, then it means that the
  initiator did not received the last issued PDU from the target. In
  this case it re-sends this PDU.
- If the target has received 1 or more ATN PDUs but the ACK PNI is not
  the target PNI - 1, then this means that this ACK is the reply of the
  previous chained I-PDU sent by the target. The target did not received
  it on the first attempt and it is being re-sent by the initiator. The
  process continues as usual.
- No ATN PDU received before this ACK PDU. This is the reply of a
  chained I-PDU. The target keeps on processing its chained I-PDU.

The code has been refactored to avoid too many indentation levels.

Also, ACK and NACK PDUs were not freed. This is now fixed.

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