wifi: mt76: mt792x: add struct mt792x_chanctx

wifi: mt76: mt792x: add struct mt792x_chanctx

We are introducing a new structure, mt792x_chanctx, to quickly identify
the linked BSS it is working on. This eliminates the need to search in
mt7921_ctx_iter() or mt7925_ctx_iter() when the channel context changes.

Co-developed-by: Sean Wang <sean.wang@mediatek.com>
Signed-off-by: Sean Wang <sean.wang@mediatek.com>
Signed-off-by: Deren Wu <deren.wu@mediatek.com>
Link: https://patch.msgid.link/20240613030241.5771-4-sean.wang@kernel.org
Signed-off-by: Felix Fietkau <nbd@nbd.name>