mac80211: fix a crash in ieee80211_beacon_get_tim on change_interface

mac80211: fix a crash in ieee80211_beacon_get_tim on change_interface

commit eb3e554b4b3a56386ef5214dbe0e3935a350178b upstream.

Some drivers (e.g. ath9k) do not always disable beacons when they're
supposed to. When an interface is changed using the change_interface op,
the mode specific sdata part is in an undefined state and trying to
get a beacon at this point can produce weird crashes.

To fix this, add a check for ieee80211_sdata_running before using
anything from the sdata.

Signed-off-by: Felix Fietkau <nbd@openwrt.org>
Signed-off-by: John W. Linville <linville@tuxdriver.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@suse.de>