mac80211: fix oops due to missing private data

This was caused by patch:

"mac80211: cooperate more with network namespaces"

The version of the patch applied doesn't match Johannes' latest:

http://johannes.sipsolutions.net/patches/kernel/all/LATEST/NNN-mac80211-netns.patch

The skb->cb virtual interface data wasn't being reset for
reuse so ath9k pooped out when trying to dereference the
private rate control info from the skb.

BUG: unable to handle kernel NULL pointer dereference
RIP: 0010:[<ffffffffa0258173>] ath_tx_rc_status+0x33/0x150 [ath9k]
<-- snip etc -->

Reported-by: Davide Pesavento <davidepesa@gmail.com>
Signed-off-by: Luis R. Rodriguez <lrodriguez@atheros.com>
Signed-off-by: John W. Linville <linville@tuxdriver.com>