mac80211: Fix for NULL pointer dereference in sta_info_get()

This addresses a NULL pointer dereference in sta_info_get().
TID and sta_info are extracted in ADDBA Timer expiry function
through the timer handler's argument.

The problem is extracging the TID (which was stored in
timer_to_tid[] array of type "u8") through "int *" typecast which
may also yield unwanted bytes for the MSB of TID that results
in incorrect sta_info and ieee80211_local pointers.

ieee80211_local pointer is NULL as illustrated below, it crashes in
sta_info_get(). The problem started when extracting ieee80211_local
pointer out of sta_info iteself and eventually crashed in
stat_info_get().

The proper way to fix is to change the data type of TID to u8
instead of u16. However changing all the occurences requires
some prototype changes as well. We should fix this in upcoming
patches.

Signed-off-by: Senthil Balasubramanian <senthilkumar@atheros.com>
Signed-off-by: Luis Rodriguez <lrodriguez@atheros.com>
Signed-off-by: John W. Linville <linville@tuxdriver.com>