qcacmn: Set local rx_desc list head to NULL

The issue sceanrio is as follows:

1) Packets are received in the rx exception ring and the
rx_descs processed are put into the pdev rx_desc freelist.

2) In the buffers replenish path, the above pdev rx_desc
freelist, on nbuf allocation or map error, is moved to the
soc rx_desc_pool freelist. The tail of the pdev rx_desc
freelist is set to NULL but not the head.

3) On receiving packets in the rx exception ring post above,
the new rx_desc are added to the pdev rx_desc freelist but
since the head of the list was not NULL prior to this, the
tail of the new list will have its next pointing to the
previous stale rx_desc list which is already moved to the soc
rx_desc_pool list.

4) In the replenish path, the buffers would be replenished
till tail of the new list but the desc_list would point
to the same aforementioned stale rx_desc list instead of NULL.
So even though replenish is successful, the desc_list check to
add the list back to soc rx_desc_pool freelist would be true.
This would cause next pointer of the tail and in effect the nbuf
pointer of the rx_desc pointed by tail to get updated.

5) On receiving the rx_desc mentioned in the previous step, nbuf
sanity check fails since nbuf address is pointing to another
rx_desc.

Fix is to set the local_desc_list also to NULL in
dp_rx_add_desc_list_to_free_list.

Change-Id: I984a4c122592547492b9d9625a71c0a90142b442
CRs-Fixed: 2704771