net: dwc_eth_qos: Prevent DMA from writing updated RX DMA descriptor

net: dwc_eth_qos: Prevent DMA from writing updated RX DMA descriptor

The DMA may attempt to write a DMA descriptor in the ring while it is
being updated. By writing the DMA descriptor buffer address to 0, it
is assured the DMA will not use such a buffer and the buffer can be
updated without any interference.

Reviewed-by: Patrick Delaunay <patrick.delaunay@st.com>
Signed-off-by: Marek Vasut <marex@denx.de>
Cc: Joe Hershberger <joe.hershberger@ni.com>
Cc: Patrice Chotard <patrice.chotard@st.com>
Cc: Patrick Delaunay <patrick.delaunay@st.com>
Cc: Ramon Fried <rfried.dev@gmail.com>
Cc: Stephen Warren <swarren@nvidia.com>