mac80211: rework MT7620 PA/LNA RF calibration

mac80211: rework MT7620 PA/LNA RF calibration

This patch makes some improvements to the MT7620 RF calibration.

1. Move MT7620 PA/LNA calibration code to dedicated functions.
2. Restore RF and BBP registers before R-Calibration.
3. Do Rx DCOC calibration again before RXIQ calibration.
4. Use SoC specific AGC initial LNA value.
5. Correct MAC_RX_EN mask in rt2800_r_calibration()[1].

[1] This change may fix the "BBP/RF register access failed" error:
ieee80211 phy0: rt2800_wait_bbp_rf_ready: Error - BBP/RF register access failed, aborting

Signed-off-by: Shiji Yang <yangshiji66@qq.com>