wifi: mac80211: add vif link addition/removal

Add the necessary infrastructure, including a new driver
method, to add/remove links to/from an interface.

Also add the missing link address to bss_conf (which we
use as link_conf too), and fill it, in station mode for
now just randomly, in AP mode we get the address from
cfg80211 since the link must be created with an address
first.

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