mac80211: check that ieee80211_set_power_mgmt only handles STA interfaces.

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.

Cc: stable@kernel.org
Signed-off-by: Benoit Papillault <benoit.papillault@free.fr>
Signed-off-by: John W. Linville <linville@tuxdriver.com>