mac80211: check that ieee80211_set_power_mgmt only handles STA interfaces.

mac80211: check that ieee80211_set_power_mgmt only handles STA interfaces.

commit e5de30c9bf4a39db9f54c4a373470ce65881ade0 upstream.

ieee80211_set_power_mgmt is meant for STA interfaces only. Moreover,
since sdata->u.mgd.mtx is only initialized for STA interfaces, using
this code for any other type of interface (like creating a monitor
interface) will result in a oops.

Signed-off-by: Benoit Papillault <benoit.papillault@free.fr>
Signed-off-by: John W. Linville <linville@tuxdriver.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@suse.de>