wifi: rt2x00: rework MT7620 PA/LNA RF calibration

1. Move MT7620 PA/LNA calibration code to dedicated functions.
2. For external PA/LNA devices, restore RF and BBP registers before
   R-Calibration.
3. Do Rx DCOC calibration again before RXIQ calibration.
4. Add some missing LNA related registers' initialization.

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/TYAP286MB0315979F92DC563019B8F238BCD4A@TYAP286MB0315.JPNP286.PROD.OUTLOOK.COM