ravb: Add gstrings_stats and gstrings_size to struct ravb_hw_info

The device stats strings for R-Car and RZ/G2L are different.

R-Car provides 30 device stats, whereas RZ/G2L provides only 15. In
addition, RZ/G2L has stats "rx_queue_0_csum_offload_errors" instead of
"rx_queue_0_missed_errors".

Add structure variables gstrings_stats and gstrings_size to struct
ravb_hw_info, so that subsequent SoCs can be added without any code
changes in the ravb_get_strings 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>
Reviewed-by: Sergei Shtylyov <sergei.shtylyov@gmail.com>
Signed-off-by: David S. Miller <davem@davemloft.net>