mt76: mt7915: fix unexpected firmware mode

Avoid firmware falling into spectrum mode since that will set
unexpected PSE/PLE thresholds which lead to Tx hang.

This mode should be cleaned before firmware download stage.

Signed-off-by: Chih-Min Chen <chih-min.chen@mediatek.com>
Signed-off-by: Ryder Lee <ryder.lee@mediatek.com>
Signed-off-by: Felix Fietkau <nbd@nbd.name>