mt76: mt7915: fix eeprom fields of txpower init values

A-die 7976 has different offset and uses different channel group
definition on txpower init values.

Fixes: 99ad32a4

 ("mt76: mt7915: add support for MT7986")
Signed-off-by: Shayne Chen <shayne.chen@mediatek.com>
Signed-off-by: MeiChia Chiu <MeiChia.Chiu@mediatek.com>
Signed-off-by: Felix Fietkau <nbd@nbd.name>