staging: brcm80211: fix for 'AC_BE txop..' logs spammed problem

Larry Finger reported this issue. The driver, under certain
conditions, spews log messages like this:

wl0: wlc_d11hdrs_mac80211: AC_BE txop exceeded phylen 159/256
 dur 1778/1504

These log messages turned out to be false alarms. Root cause was
that the AC was derived from the sk_buff::priority field. Fix was
to derive the AC from the sk_buff using skb_get_queue_mapping()

Signed-off-by: Roland Vossen <rvossen@broadcom.com>
Tested-by: Larry Finger <Larry.Finger@lwfinger.net>
Acked-by: Larry Finger <Larry.Finger@lwfinger.net>
Signed-off-by: Greg Kroah-Hartman <gregkh@suse.de>