mlxsw: spectrum_ipip: Remove overlay protocol from can_offload() callback

The overlay protocol (i.e., IPv4/IPv6) that is being encapsulated has
no impact on whether a certain IP tunnel can be offloaded or not. Only
the underlay protocol matters.

Therefore, remove the unused overlay protocol parameter from the
callback.

This will later allow us to consolidate code paths between IPv4 and IPv6
code.

Signed-off-by: Ido Schimmel <idosch@nvidia.com>
Reviewed-by: Jiri Pirko <jiri@nvidia.com>
Signed-off-by: Jakub Kicinski <kuba@kernel.org>