wifi: rt2x00: fix MT7620 low RSSI issue

On Mediatek vendor driver[1], MT7620 (RT6352) uses different RSSI
base value '-2' compared to the other RT2x00 chips. This patch
introduces the SoC specific base value to fix the low RSSI value
reports on MT7620.

[1] Found on MT76x2E_MT7620_LinuxAP_V3.0.4.0_P3 ConvertToRssi().

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/TYAP286MB031571CDB146C414A908A66DBCFEA@TYAP286MB0315.JPNP286.PROD.OUTLOOK.COM