cfg80211: Use 5MHz bandwidth by default when checking usable channels

Current code checks if the 20MHz bandwidth is allowed for
particular channel -- if it is not, the channel is disabled.
Since we need to use 5/10 MHz channels, this code is modified in
the way that the default bandwidth to check is 5MHz. If the
maximum bandwidth allowed by the channel is smaller than 5MHz,
the channel is disabled. Otherwise the channel is used and the
flags are set according to the bandwidth allowed by the channel.

Signed-off-by: Rostislav Lisovy <rostislav.lisovy@fel.cvut.cz>
Signed-off-by: Johannes Berg <johannes.berg@intel.com>