The commit 42624d49
created following sparse warning
>net/mac80211/sta_info.c:965:24: warning: incorrect type in assignment (different address spaces)
>net/mac80211/sta_info.c:965:24:    expected struct tid_ampdu_tx *tid_tx
>net/mac80211/sta_info.c:965:24:    got struct tid_ampdu_tx [noderef] <asn:4>*<noident>

Making use of rcu_dereference_protected to fix the problem.

V2:
 - Replacing rcu_dereference with rcu_dereference_protected
   as suggested by Johannes.
 - Adding mutex_lock/unlock to satisfy the condition at
   rcu_dereference_protected

Cc: Nishant Sarmukadam <nishants@marvell.com>
Reported-by: NJohannes Berg <johannes@sipsolutions.net>
Signed-off-by: NYogesh Ashok Powar <yogeshp@marvell.com>
Signed-off-by: NJohn W. Linville <linville@tuxdriver.com>