ravb: fix warning about memcpy length

This fixes sparse warning:

ravb_main.c:1257 ravb_get_strings() error: memcpy() '*ravb_gstrings_stats' too small (32 vs 960)

Signed-off-by: Niklas Söderlund <niklas.soderlund+renesas@ragnatech.se>
Reviewed-by: Geert Uytterhoeven <geert+renesas@glider.be>
Acked-by: Sergei Shtylyov <sergei.shtylyov@cogentembedded.com>
Signed-off-by: David S. Miller <davem@davemloft.net>