mac80211: fix wme code

In commit e100bb64 (mac80211:
QoS related cleanups) I accidentally changed a variable from
int to u16 causing a warning that a comparison for < 0 was always
false. John thought this was a missing deletion of code and removed
the warning by deleting the never executed branch of code in commit
3df5ee60

 (wireless: fix warning
introduced by "mac80211: QoS related cleanups") but the problem really
was my mistake of using a u16 variable for the queue variable when
that variable can also contain an error code. This patch restores
the original code and variable type.

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