wifi: rt2x00: rework MT7620 channel config function

1. Move the channel configuration code from rt2800_vco_calibration()
   to the rt2800_config_channel().
2. Use MT7620 SoC specific AGC initial LNA value instead of the
   RT5592's value.
3. BBP{195,196} pairing write has been replaced with
   rt2800_bbp_glrt_write() to reduce redundant code.

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/TYAP286MB0315622A4340BFFA530B1B86BCD4A@TYAP286MB0315.JPNP286.PROD.OUTLOOK.COM