wil6210: optimize wil_release_reorder_frames

In case of receiving frame with sequence number far greater than current,
wil_release_reorder_frames() will iterate many times over empty buffer.

Optimize this case by checking buffer emptiness and simply update
head_seq_num without iterating.

Suggested-by: Vladimir Shulman <Vladimir.Shulman@Wilocity.com>
Signed-off-by: Vladimir Kondratiev <qca_vkondrat@qca.qualcomm.com>
Signed-off-by: John W. Linville <linville@tuxdriver.com>