mac80211: fix use before check of Qdisc length

This patch fixes use of Qdisc length in requeue function, before we checked
the reference is valid. (Adrian Bunk's catch)

Signed-off-by: Ron Rindjunsky <ron.rindjunsky@intel.com>
Signed-off-by: Adrian Bunk <bunk@kernel.org>
Signed-off-by: John W. Linville <linville@tuxdriver.com>