qcacmn: Allow drop pkts when pkts pending is large

On some cases, dp_rx_thread can not handle nbufs in time,
then a large number of packets are pending in
rx_thread->nbuf_queue, which run out of system memory at
last and failed to malloc new buffers for refill buffer
ring, fw crash is encountered. To avoid this, drop RX
packets when the pending number becomes large.

Change-Id: I370ad983b185b6ecb28fa7c0b4820b8edc00c710
CRs-Fixed: 2737191