mlxsw: spectrum_ipip: Rename Spectrum-2 ip6gre operations

There are two main differences between Spectrum-1 and newer ASICs in
terms of IP-in-IP support:

1. In Spectrum-1, RIFs representing ip6gre tunnels require two entries
   in the RIF table.

2. In Spectrum-2 and newer ASICs, packets ingress the underlay (during
   encapsulation) and egress the underlay (during decapsulation) via a
   special generic loopback RIF.

The first difference was handled in previous patches by adding the
'double_rif_entry' field to the Spectrum-1 operations structure of
ip6gre RIFs. The second difference is handled during RIF creation, by
only creating a generic loopback RIF in Spectrum-2 and newer ASICs.

Therefore, the ip6gre operations can be shared between Spectrum-1 and
newer ASIC in a similar fashion to how the ipgre operations are shared.

Rename the operations to not be Spectrum-2 specific and move them
earlier in the file so that they could later be used for Spectrum-1.

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>