nl80211: Add stop AP mode event API

nl80211: Add stop AP mode event API

Stop AP command can be used by the driver as an event to indicate that
AP mode has stopped operation. WLAN driver may have encountered errors
that has forced the driver to report this event or concurrent operations
on virtual interfaces may have forced AP operation to be stopped. When
in P2P GO mode, wpa_supplicant uses this event to remove P2P group to
keep in sync with the driver state.

Signed-hostap: Jouni Malinen <jouni@qca.qualcomm.com>