mbssid: Functions for building Multiple BSSID elements

mbssid: Functions for building Multiple BSSID elements

Add Multiple BSSID element data per IEEE Std 802.11ax-2021, 9.4.2.45.
Split the BSSes into multiple elements if the data does not fit in
the 255 bytes allowed for a single element.

Store the total count of elements created and the offset to the start
of each element in the provided buffer.

Set the DTIM periods of non-transmitted profiles equal to the EMA
profile periodicity if those are not a multiple of the latter already as
recommended in IEEE Std 802.11ax-2021, Annex AA (Multiple BSSID
configuration examples).

Signed-off-by: Aloka Dixit <quic_alokad@quicinc.com>
Co-developed-by: John Crispin <john@phrozen.org>
Signed-off-by: John Crispin <john@phrozen.org>