net: ethernet: mtk_eth_soc: remove unused txd_pdma pointer in mtk_xdp_submit_frame

Get rid of unnecessary txd_pdma pointer in mtk_xdp_submit_frame for loop
since it is actually used at the end of the routine using latest mtk_tx_dma
consumed pointer as reference.

Signed-off-by: Lorenzo Bianconi <lorenzo@kernel.org>
Link: https://lore.kernel.org/r/2c40b0fbb9163a0d62ff897abae17db84a9f3b99.1660669138.git.lorenzo@kernel.org
Signed-off-by: Jakub Kicinski <kuba@kernel.org>