wifi: mt76: mt7925: add test mode support

The test mode interface allows controlled execution of chip-level
operations such as continuous transmission, reception tests, and
register access, which are essential during bring-up, diagnostics,
and factory testing.

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/20250505233618.1951021-2-sean.wang@kernel.org


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