wifi: mt76: mt7921e: add pci .shutdown() support

Some combinations of hosts cannnot detect mt7921e after reboot. The
interoperability issue is caused by the status mismatch between host
and chip fw. In such cases, the driver should stop chip activities
and reset chip to default state before reboot.

Suggested-by:  <angelogioacchino.delregno@collabora.com>
Co-developed-by: Deren Wu <deren.wu@mediatek.com>
Signed-off-by: Deren Wu <deren.wu@mediatek.com>
Signed-off-by: Leon Yen <Leon.Yen@mediatek.com>
Signed-off-by: Felix Fietkau <nbd@nbd.name>