cfg80211:: fix possible NULL pointer dereference

In cfg80211_inform_bss_frame() wiphy is first dereferenced on privsz
initialisation and then it is checked for NULL. This patch fixes that.

Signed-off-by: Mariusz Kozlowski <mk@lab.zgora.pl>
Acked-by: Johannes Berg <johannes@sipsolutions.net>
Signed-off-by: John W. Linville <linville@tuxdriver.com>