cfg80211: fix race in giwrate

cfg80211_wext_giwrate doesn't lock the wdev, so it
cannot access current_bss race-free. Also, there's
little point in trying to ask the driver for an AP
that it never told us about, so avoid that case.

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