Check a STA is associated before regulating its PS-status in mac80211.
Should never happen, so warn as a precaution.

[Small cosmetic change wrt Kalle Valo's comment. -- Luca]
Signed-off-by: Arik Nemtsov <arik@wizery.com>
Signed-off-by: Luciano Coelho <coelho@ti.com>