mlxsw: spectrum_ipip: Add Spectrum-1 ip6gre support

As explained in the previous patch, the existing Spectrum-2 ip6gre
implementation can be reused for Spectrum-1. Change the Spectrum-1
ip6gre operations structure to use the common operations.

Signed-off-by: Ido Schimmel <idosch@nvidia.com>
Reviewed-by: Amit Cohen <amcohen@nvidia.com>
Signed-off-by: Petr Machata <petrm@nvidia.com>
Signed-off-by: Jakub Kicinski <kuba@kernel.org>