net80211: HT: check for feature support in ht_recv_action_ht_txchwidth()

ht_recv_action_ht_txchwidth() can blindly change the channel width to
40 Mhz whether or not that is supported. If 20/40 is not supported
there is nothing to do as the channel width cannot change in that case.

While here mark unused arguments with __unused.

Sponosred by: The FreeBSD Foundation
MFC adter: 3 days
Reviewed by: adrian
Differential Revision: https://reviews.freebsd.org/D47857