wifi: mt76: mt7603: initialize chainmask

Fixes reported tx power by accounting for the combined output

Link: https://patch.msgid.link/20240827093011.18621-3-nbd@nbd.name
Signed-off-by: Felix Fietkau <nbd@nbd.name>

drivers/net/wireless/mediatek/mt76/mt7603/eeprom.c

@@ -181,6 +181,7 @@ int mt7603_eeprom_init(struct mt7603_dev *dev)
 	    is_mt7688(dev))
 		dev->mphy.antenna_mask = 1;
 
+	dev->mphy.chainmask = dev->mphy.antenna_mask;
 	mt76_eeprom_override(&dev->mphy);
 
 	return 0;