ath9k: Null out references to stale pointers.

This doesn't fix any problem that I'm aware of, but should
make it harder to add use-after-free type bugs in the
future.

Signed-off-by: Ben Greear <greearb@candelatech.com>
Signed-off-by: John W. Linville <linville@tuxdriver.com>

diff --cc drivers/net/wireless/ath/ath9k/beacon.c
Simple merge

diff --cc drivers/net/wireless/ath/ath9k/recv.c
Simple merge

diff --cc drivers/net/wireless/ath/ath9k/xmit.c
Simple merge