cfg80211: fix refcount leak

The code in cfg80211's cfg80211_bss_update erroneously
grabs a reference to the BSS, which means that it will
never be freed.

Signed-off-by: Johannes Berg <johannes@sipsolutions.net>
Cc: stable@kernel.org [2.6.29, 2.6.30]
Signed-off-by: John W. Linville <linville@tuxdriver.com>