rc80211-pid: fix sta_info refcounting

Fix a bug which caused uncorrect refcounting of PHYs in mac80211. Thanks to
Johannes Berg for spotting this out.

Cc: Johannes Berg <johannes@sipsolutions.net>
Signed-off-by: Stefano Brivio <stefano.brivio@polimi.it>
Signed-off-by: John W. Linville <linville@tuxdriver.com>
Signed-off-by: David S. Miller <davem@davemloft.net>