net: sun8i_emac: Improve cache maintenance on RX descriptor init

net: sun8i_emac: Improve cache maintenance on RX descriptor init

Before we initialise the RX descriptors, there is no need to *clean*
them from the cache, as we touch them for the first time.
However we should cover the case that those buffers contain dirty cache
lines, which could be evicted and written back to DRAM any time later,
in the worst case *after* the MAC has transferred a packet into them.

Signed-off-by: Andre Przywara <andre.przywara@arm.com>
Acked-by: Maxime Ripard <mripard@kernel.org>
Tested-by: Amit Singh Tomar <amittomer25@gmail.com> # Pine64+
Reviewed-by: Jagan Teki <jagan@amarulasolutions.com>