mac80211: rt2x00: some improvements for mt7620

mac80211: rt2x00: some improvements for mt7620

This patch contains some performance and stability improvements
for MT7620:

1. Always calibrate MT7620 when switching channel.
2. Rework link tuner for MT7620.
3. Correct MT7620 SDM mode register value.
4. Fix register operation on RXIQ calibration.
5. Fix TX_PIN_CFG and TX_ALC_VGA3 init values.

Signed-off-by: Shiji Yang <yangshiji66@qq.com>
Link: https://github.com/openwrt/openwrt/pull/16845
Signed-off-by: Hauke Mehrtens <hauke@hauke-m.de>
(cherry picked from commit 5d583901657bdfbbf9fad77d9247872427aa5c99)