wifi: mt76: mt7925: extend MCU support for testmode

Add MCU command and its handling needed for testmode support on
MT7925. This enables low-level chip testing features such as
continuous TX/RX..

Co-developed-by: Sean Wang <sean.wang@mediatek.com>
Signed-off-by: Sean Wang <sean.wang@mediatek.com>
Signed-off-by: Michael Lo <michael.lo@mediatek.com>
Link: https://patch.msgid.link/20250505233618.1951021-1-sean.wang@kernel.org


Signed-off-by: Felix Fietkau <nbd@nbd.name>