net: sun8i_emac: Reduce cache maintenance on TX descriptor init

When we initialise the TX descriptors, there is no need yet to clean
them all to memory, as they don't contain any data yet. Later we will
touch and clean each descriptor anyway.
However we tell the MAC about the beginning of the chain, so we have to
clean at least the first descriptor, to make it clear that this is empty
and there are no packets to transfer yet.

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>