mt76: mt7615: sdio code must access rate/key regs in preocess context

As usb, sdio relies on mt76 workqueue to configure tx rate or upload
keys to the hw. This is a preliminary patch to add SDIO support to
mt76 driver

Co-developed-by: Sean Wang <sean.wang@mediatek.com>
Signed-off-by: Sean Wang <sean.wang@mediatek.com>
Signed-off-by: Lorenzo Bianconi <lorenzo@kernel.org>
Signed-off-by: Felix Fietkau <nbd@nbd.name>