net: ethtool: let drivers declare max size of RSS indir table and key

Some drivers (bnxt but I think also mlx5 from ML discussions) change
the size of the indirection table depending on the number of Rx rings.
Decouple the max table size from the size of the currently used table,
so that we can reserve space in the context for table growth.

Static members in ethtool_ops are good enough for now, we can add
callbacks to read the max size more dynamically if someone needs
that.

Reviewed-by: Pavan Chebbi <pavan.chebbi@broadcom.com>
Link: https://patch.msgid.link/20240711220713.283778-3-kuba@kernel.org


Signed-off-by: Jakub Kicinski <kuba@kernel.org>