net: ethernet: mtk_wed: rename mtk_rxbm_desc in mtk_wed_bm_desc

Rename mtk_rxbm_desc structure in mtk_wed_bm_desc since it will be used
even on tx side by MT7988 SoC.

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