wifi: mt76: connac: add support for dsp firmware download

Add FW_START_WORKING_PDA_DSP for the indication of starting DSP
firmware download, which is for phy-related control.
The firmware is transparent to the driver, but it's necessary for the
firmware download process.

Reviewed-by: Shayne Chen <shayne.chen@mediatek.com>
Signed-off-by: Peter Chiu <chui-hao.chiu@mediatek.com>
Signed-off-by: Shayne Chen <shayne.chen@mediatek.com>
Signed-off-by: Felix Fietkau <nbd@nbd.name>