ravb: Factorise ravb_ring_init function

The ravb_ring_init function uses an extended descriptor in RX for
R-Car and normal descriptor for RZ/G2L. Add a helper function
for RX ring buffer allocation to support 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>