cfg80211: copy hold when replacing BSS

When we receive a probe response frame we can replace the
BSS struct in our list -- but if that struct is held then
we need to hold the new one as well.

We really should fix this completely and not replace the
struct, but this is a bandaid for now.

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