wifi: rt2x00: add RF self TXDC calibration for MT7620

Add TX self calibration based on mtk driver.

Signed-off-by: Tomislav Požega <pozega.tomislav@gmail.com>
Signed-off-by: Daniel Golle <daniel@makrotopia.org>
Acked-by: Stanislaw Gruszka <stf_xl@wp.pl>
Signed-off-by: Kalle Valo <kvalo@kernel.org>
Link: https://lore.kernel.org/r/dbb6e5a0c12d6101477bd09e83253091d21512c9.1663445157.git.daniel@makrotopia.org