net: ethernet: mtk_wed: refactor mtk_wed_check_wfdma_rx_fill routine

Refactor mtk_wed_check_wfdma_rx_fill() in order to be reused adding HW
receive offload support for MT7988 SoC.

Co-developed-by: Sujuan Chen <sujuan.chen@mediatek.com>
Signed-off-by: Sujuan Chen <sujuan.chen@mediatek.com>
Signed-off-by: Lorenzo Bianconi <lorenzo@kernel.org>
Signed-off-by: Paolo Abeni <pabeni@redhat.com>