wifi: rt2x00: improve MT7620 register initialization

1. Do not hard reset the BBP. We can use soft reset instead. This
   change has some help to the calibration failure issue.
2. Enable falling back to legacy rate from the HT/RTS rate by
   setting the HT_FBK_TO_LEGACY register.
3. Implement MCS rate specific maximum PSDU size. It can improve
   the transmission quality under the low RSSI condition.
4. Set BBP_84 register value to 0x19. This is used for extension
   channel overlapping IOT.

Signed-off-by: Shiji Yang <yangshiji66@outlook.com>
Acked-by: Stanislaw Gruszka <stf_xl@wp.pl>
Signed-off-by: Kalle Valo <kvalo@kernel.org>
Link: https://lore.kernel.org/r/TYAP286MB031553CCD4B7A3B89C85935DBCD4A@TYAP286MB0315.JPNP286.PROD.OUTLOOK.COM