ravb: Factorise ravb_ring_free function

R-Car uses extended descriptor in RX, whereas RZ/G2L uses normal
descriptor. Factorise ravb_ring_free function so that it can
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>