net80211 amdpu: Simplify a few loops that drain an mbufq

These loops already handled a NULL return from mbufq_dequeue when the
queue was empty, so remove a redundant check of mbufq_len before
dequeueing.

Reviewed by: bz
Sponsored by: Chelsio Communications
Differential Revision: https://reviews.freebsd.org/D43336

(cherry picked from commit 6977311633c73f594ff3dae150d6d1fe06105a8f)