wifi: mt76: mt7996: fix chainmask calculation in mt7996_set_antenna()

Fix per-band chainmask when restoring from the dev chainmask.

Fixes: 98686cd2 ("wifi: mt76: mt7996: add driver for MediaTek Wi-Fi 7 (802.11be) devices")
Signed-off-by: Shayne Chen <shayne.chen@mediatek.com>
Signed-off-by: Felix Fietkau <nbd@nbd.name>