cfg80211: Dynamic channel bandwidth changes in AP mode

This extends NL80211_CMD_SET_CHANNEL to allow dynamic channel bandwidth
changes in AP mode (including P2P GO) during a lifetime of the BSS. This
can be used to implement, e.g., HT 20/40 MHz co-existence rules on the
2.4 GHz band.

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