mac80211: use just spin_lock() in ieee80211_get_tkip_p2k()

ieee80211_get_tkip_p2k() may be called with interrupts
disabled, so spin_unlock_bh() isn't safe and leads to
warnings. Since it's always called with BHs disabled
already, just use spin_lock().

Cc: stable@vger.kernel.org
Reported-by: Milan Kocian <milon@wq.cz>
Acked-by: Stanislaw Gruszka <sgruszka@redhat.com>
Signed-off-by: Johannes Berg <johannes.berg@intel.com>