AP: Allow starting multiple interfaces within single MLD

AP: Allow starting multiple interfaces within single MLD

Add support for including multiple hostapd interfaces in the same AP
MLD, i.e., all using the same underlying driver network interface.

To do so, when a new hostapd interface is added, if there is already
another interface using the same underlying network interface, associate
the new interface with the same private data object, instead of creating
a new one.

As some of the BSSs are non-first BSSs, meaning that they reuse the
drv_priv of the initial BSS, make sure not to double free it.

Currently multiple BSS entries are not supported so always use bss[0]
for MLD.

Signed-off-by: Andrei Otcheretianski <andrei.otcheretianski@intel.com>
Signed-off-by: Ilan Peer <ilan.peer@intel.com>