cfg80211: keep track of BSSes

In order to avoid problems with BSS structs going away
while they're in use, I've long wanted to make cfg80211
keep track of them. Without the SME, that wasn't doable
but now that we have the SME we can do this too. It can
keep track of up to four separate authentications and
one association, regardless of whether it's controlled
by the cfg80211 SME or the userspace SME.

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