wifi: cfg80211: add optional link add/remove callbacks

Add some optional callbacks for link add/remove so that
drivers can react here. Initially, I thought it would be
sufficient to just create the link in start_ap etc., but
it turns out that's not so simple, since there are quite
a few callbacks that can be called: if they're erroneously
without start_ap, things might crash.

Thus it might be easier for drivers to allocate all the
necessary data structures immediately, to not have to
worry about it in each callback, since cfg80211 checks
that the link ID is valid (has been added.)

Signed-off-by: Johannes Berg <johannes.berg@intel.com>