net: ethernet: mtk_wed: make memory region optional

Make mtk_wed_wo_memory_region optionals.
This is a preliminary patch to introduce Wireless Ethernet Dispatcher
support for MT7988 SoC since MT7988 WED fw image will have a different
layout.

Signed-off-by: Lorenzo Bianconi <lorenzo@kernel.org>
Signed-off-by: Paolo Abeni <pabeni@redhat.com>