hostap: pad the skb->cb usage in lieu of a proper fix

Like mac80211 did, this driver makes 'clever' use of skb->cb to pass
information along with an skb as it is requeued from the virtual device
to the physical wireless device.  Unfortunately, that trick no longer
works...

Unlike mac80211, code complexity and driver apathy makes this hack
the best option we have in the short run.  Hopefully someone will
eventually be motivated to code a proper fix before all the effected
hardware dies.

(Above text by me.  Johannes officially disavows all knowledge of this
hack. -- JWL)

Signed-off-by: Johannes Berg <johannes@sipsolutions.net>
Cc: stable@kernel.org
Signed-off-by: John W. Linville <linville@tuxdriver.com>