Port iwm_enable_ac_txq() from OpenBSD. It's effictively the
command FIFO path in the existing iwm_enable_txq().
The OpenBSD driver does a bunch of TX aggregation queue setup in
iwm_enable_txq() which isn't relevant here just quite yet and
it doesn't currently hurt to leave the existing one here
whilst I figure out the next steps.
Obtained from: OpenBSD a6e25c727a2214219715b51ba6af2221d7e0d674