net: dwc_eth_qos: Split TX and RX DMA rings

Separate TX and RX DMA rings to make their handling slightly clearer.
This is a preparatory patch for bulk RX descriptor flushing.

Signed-off-by: Marek Vasut <marex@denx.de>
Reviewed-by: Patrice Chotard <patrice.chotard@foss.st.com>
Reviewed-by: Ramon Fried <rfried.dev@gmail.com>