ravb: Factorise ravb_ring_format function

The ravb_ring_format function uses an extended descriptor in RX
for R-Car compared to the normal descriptor for RZ/G2L. Factorise
RX ring buffer buildup to extend the support for later SoC.

Signed-off-by: Biju Das <biju.das.jz@bp.renesas.com>
Reviewed-by: Lad Prabhakar <prabhakar.mahadev-lad.rj@bp.renesas.com>
Signed-off-by: David S. Miller <davem@davemloft.net>