mac80211: fix-up build breakage in 2.6.33

"mac80211: fix skb buffering issue" is based on what will become 2.6.34,
so it includes an incompatible usage of sta_info_get.  This patch will
need to be effectively reverted when merging for 2.6.34.

Signed-off-by: John W. Linville <linville@tuxdriver.com>