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

drivers/net/wireless/ralink/rt2x00/rt2800lib.c

@@ -856,6 +856,7 @@ static int rt2800_agc_to_rssi(struct rt2x00_dev *rt2x00dev, u32 rxwi_w2)
 	s8 rssi0 = rt2x00_get_field32(rxwi_w2, RXWI_W2_RSSI0);
 	s8 rssi1 = rt2x00_get_field32(rxwi_w2, RXWI_W2_RSSI1);
 	s8 rssi2 = rt2x00_get_field32(rxwi_w2, RXWI_W2_RSSI2);
+	s8 base_val = rt2x00_rt(rt2x00dev, RT6352) ? -2 : -12;
 	u16 eeprom;
 	u8 offset0;
 	u8 offset1;
@@ -880,9 +881,9 @@ static int rt2800_agc_to_rssi(struct rt2x00_dev *rt2x00dev, u32 rxwi_w2)
 	 * If the value in the descriptor is 0, it is considered invalid
 	 * and the default (extremely low) rssi value is assumed
 	 */
-	rssi0 = (rssi0) ? (-12 - offset0 - rt2x00dev->lna_gain - rssi0) : -128;
-	rssi1 = (rssi1) ? (-12 - offset1 - rt2x00dev->lna_gain - rssi1) : -128;
-	rssi2 = (rssi2) ? (-12 - offset2 - rt2x00dev->lna_gain - rssi2) : -128;
+	rssi0 = (rssi0) ? (base_val - offset0 - rt2x00dev->lna_gain - rssi0) : -128;
+	rssi1 = (rssi1) ? (base_val - offset1 - rt2x00dev->lna_gain - rssi1) : -128;
+	rssi2 = (rssi2) ? (base_val - offset2 - rt2x00dev->lna_gain - rssi2) : -128;
 
 	/*
 	 * mac80211 only accepts a single RSSI value. Calculating the