net: mt7531: ensure all MACs are powered down before reset

The datasheet [1] explicit describes it as requirement for a reset.

[1] MT7531 Reference Manual for Development Board rev 1.0, page 735

Signed-off-by: Alexander Couzens <lynxis@fe80.eu>
Signed-off-by: Jakub Kicinski <kuba@kernel.org>