qcacld-3.0: Fix wrong calculation of DP thread stats

In FISA path number of list elements in head nbuf is not updated,
leading to wrong calculation of dp thread stats and packets are dropped.
Update the number of list elements in head nbuf before flush.

During calculation of outstanding packets to be processed
by dp rx thread take rx flushed packets into consideration.

Change-Id: Id96ceee71e035fdbe17f0da4b662c6f848296fbd
CRs-Fixed: 2811001