cfg80211: fix nl80211 frequency handling

Fix two small bugs with HT frequency setting:
 * HT is accepted even when the driver is incapable
 * HT40 is accepted when the driver cannot do 40 MHz
 (both on the selected band)

Also simplify the code a little.

Signed-off-by: Johannes Berg <johannes@sipsolutions.net>
Signed-off-by: John W. Linville <linville@tuxdriver.com>