rt2800: fix LNA gain assignment for MT7620

The base value used for MT7620 differs from Rt5392 which resulted in
quite bad RX signal quality. Fix this by using the correct base value as
well as the LNA calibration values for HT20.

Reported-by: Tom Psyborg <pozega.tomislav@gmail.com>
Signed-off-by: Daniel Golle <daniel@makrotopia.org>
Signed-off-by: Kalle Valo <kvalo@codeaurora.org>