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>