ravb: Add stats_len to struct ravb_hw_info

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".

Replace RAVB_STATS_LEN macro with a structure variable stats_len to
struct ravb_hw_info, to support subsequent SoCs without any code changes
to the ravb_get_sset_count 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: Sergei Shtylyov <sergei.shtylyov@gmail.com>
Signed-off-by: David S. Miller <davem@davemloft.net>