net: dwc_eth_qos: Flush the RX descriptors on init

net: dwc_eth_qos: Flush the RX descriptors on init

Currently the code only flushes the first RX descriptor, not every entry
in the RX descriptor ring. Fix this, to make sure the DMA engine can pick
the RX descriptors correctly.

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>