wifi: mt76: mt7996: enable BSS_CHANGED_BASIC_RATES support

The connac3 removes fixed rate fields to reduce txd size and introduces
global rate tables (64 entries) for rate setting. Driver needs to fill
the corresponding idx in MT_TXD6_TX_RATE while tx, and push mt76_rate
into predifined table at bootup stage so that  mvif->basic_rates_idx
can immediately switch out once setting changes.

spe_idx is also needed for fixed rate frames, and will be updated by
future patches.

Note that all table entries are shared across driver and firmware
(i.e.TxBF), hence adding MT7996_BASIC_RATES_TBL to reflect mapping
status.

Signed-off-by: Ryder Lee <ryder.lee@mediatek.com>
Signed-off-by: Felix Fietkau <nbd@nbd.name>