wifi: mt76: mt7996: fix insecure data handling of mt7996_mcu_ie_countdown()

Coverity message:
using tainted "hdr->band" variable as an index into an array "(*dev).mt76.phys".

Reported-by: coverity-bot <keescook+coverity-bot@chromium.org>
Addresses-Coverity-ID: 1527797 ("Insecure data handling")
Fixes: 98686cd2

 ("wifi: mt76: mt7996: add driver for MediaTek Wi-Fi 7 (802.11be) devices")
Signed-off-by: Ryder Lee <ryder.lee@mediatek.com>
Signed-off-by: Felix Fietkau <nbd@nbd.name>