ravb: Add max_rx_len to struct ravb_hw_info

The maximum descriptor size that can be specified on the reception side for
R-Car is 2048 bytes, whereas for RZ/G2L it is 8096.

Add the max_rx_len variable to struct ravb_hw_info for allocating different
RX skb buffer sizes for R-Car and RZ/G2L using the netdev_alloc_skb
function.

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