net: ethernet: mtk_eth_soc: move tx dma desc configuration in mtk_tx_set_dma_desc

Move tx dma descriptor configuration in mtk_tx_set_dma_desc routine.
This is a preliminary patch to introduce mt7986 ethernet support since
it relies on a different tx dma descriptor layout.

Tested-by: Sam Shih <sam.shih@mediatek.com>
Signed-off-by: Lorenzo Bianconi <lorenzo@kernel.org>
Signed-off-by: David S. Miller <davem@davemloft.net>