mac80211: fix a crash in ieee80211_beacon_get_tim on change_interface

mac80211: fix a crash in ieee80211_beacon_get_tim on change_interface

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 <[email protected]>
Cc: [email protected]
Signed-off-by: John W. Linville <[email protected]>