cfg80211: regulatory: handle 5 and 10 MHz channels properly

The original assumption of 20MHz wide channels hasn't been true since
the addition of support for 5 and 10 MHz channels.
Change the code to no longer disable all channels that don't fit into
the 20MHz grid, but instead set the appropriate flags to disable
operation on specific bandwidths.

Signed-off-by: Matthias May <matthias.may@neratec.com>
[reword commit message]
Signed-off-by: Johannes Berg <johannes.berg@intel.com>