wl18xx: don't use MIMO when ht_mode is set to wide

If the wl18xx module is loaded with ht_mode=wide (the default), we
shouldn't use MIMO rates when the channel type is not HT40.  Fix this
by checking the ht_mode before deciding which rates to used.
Additionally, set the ht_mode parameter explicitly to "wide" as the
default.

Signed-off-by: Luciano Coelho <coelho@ti.com>
Signed-off-by: Arik Nemtsov <arik@wizery.com>