net: ethernet: mtk_eth_soc: increase tx ring size for QDMA devices

In order to use the hardware traffic shaper feature, a larger tx ring is
needed, especially for the scratch ring, which the hardware shaper uses to
reorder packets.

Signed-off-by: Felix Fietkau <nbd@nbd.name>
Link: https://lore.kernel.org/r/20221116080734.44013-2-nbd@nbd.name


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