iwm: Enable all 31 tx queues.

For now iwm only ever uses queue 0 and the management queue, but my 9560
raises a software error interrupt during initialization if this flag is
not set. iwlwifi sets it for all 7000- and 8000-series hardware, so we
might as well do it unconditionally.

MFC after: 2 weeks
Sponsored by: The FreeBSD Foundation