MFC r321508:

[iwm] Cleanup mbufq draining. Add iwm_xmit_queue_drain() like in iwn(4).

• iwm_xmit_queue_drain() calls ieee80211_free_node(), removing a possible memory leak, compared to using just mbufq_drain().

• Remove duplicate mbufq_drain() from iwm_mvm_rm_sta(), this should be handled in the caller.