mac80211: Add support for connection monitor in hardware

This patch is based on a RFC patch by Kalle Valo.

The wl1271 has a feature which handles the connection monitor logic
in hardware, basically sending periodically nullfunc frames and reporting
to the host if AP is lost, after attempting to recover by sending
probe-requests to the AP.

Add support to mac80211 by adding a new flag IEEE80211_HW_CONNECTION_MONITOR
which prevents conn_mon_timer from triggering during idle periods, and
prevents sending probe-requests to the AP if beacon-loss is indicated by the
hardware.

Cc: Kalle Valo <kalle.valo@nokia.com>
Signed-off-by: Juuso Oikarinen <juuso.oikarinen@nokia.com>
Reviewed-by: Johannes Berg <johannes@sipsolutions.net>
Signed-off-by: John W. Linville <linville@tuxdriver.com>