net: ethernet: mtk_eth_soc: drop packets to WDMA if the ring is full

Improves handling of DMA ring overflow.
Clarify other WDMA drop related comment.

Signed-off-by: Felix Fietkau <nbd@nbd.name>
Link: https://lore.kernel.org/r/20221116080734.44013-3-nbd@nbd.name


Signed-off-by: Jakub Kicinski <kuba@kernel.org>