mt76: mt7921: move mt76_connac_mcu_set_hif_suspend to bus-related files

This is a preliminary patch for the following patch
("mt76: mt7921s: fix the device cannot sleep deeply in suspend).

mt76_connac_mcu_set_hif_suspend eventually would be handled in each
bus-level suspend/resume handler in either mt7921/sdio.c or mt7921/pci.c
depending on what type of the bus the device is running on. We can move
mt76_connac_mcu_set_hif_suspend to bus-related files to simplify the logic.

Signed-off-by: Sean Wang <sean.wang@mediatek.com>
Signed-off-by: Felix Fietkau <nbd@nbd.name>