mac80211: remove misleading 'res' variable

When this function returns != CONTINUE, it needs to put the
station struct it has acquired. Hence, having this unused
variable is not just superfluous but also misleading.

Signed-off-by: Johannes Berg <johannes@sipsolutions.net>
Signed-off-by: John W. Linville <linville@tuxdriver.com>
Signed-off-by: David S. Miller <davem@davemloft.net>