mac80211: Support dynamic AP mode channel width changes

Implement the new cfg80211 capability to enable mac80211-based drivers
to support for dynamic channel bandwidth changes (e.g., HT 20/40 MHz
changes).

Signed-off-by: Jouni Malinen <jouni@qca.qualcomm.com>
Signed-off-by: Johannes Berg <johannes.berg@intel.com>