mt76: mt7915: fix MT_EE_CAL_GROUP_SIZE

Fix wrong offset for pre-calibration data.

Fixes: 495184ac ("mt76: mt7915: add support for applying pre-calibration data")
Signed-off-by: Ryder Lee <ryder.lee@mediatek.com>
Signed-off-by: Felix Fietkau <nbd@nbd.name>