The supported bandwidth field is a two-bit field, not a bitmap,
so treat it accordingly when disabling 80+80 or 160 MHz.

Note that we can only advertise "80+80 and 160" or "160", not
"80+80" by itself, so disabling 160 also disables 80+80.
Signed-off-by: NJohannes Berg <johannes.berg@intel.com>