mt76: mt7921s: clear MT76_STATE_MCU_RUNNING immediately after reset

clear the flag MT76_STATE_MCU_RUNNING immediately after reset to indicate
the MCU has already stopped working at the point. That is a preliminary
patch for the following patch to perform the register access in the
remaining reset handler using SDIO mailbox way instead of MCU command
because the RAM firmware is cleared out.

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