ravb: Factorise ravb_rx function

R-Car uses an extended descriptor in RX whereas, RZ/G2L uses
normal descriptor in RX. Factorise the ravb_rx function to
support the 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>