mlxsw: spectrum_ipip: Create common function for mlxsw_sp_ipip_ol_netdev_change_gre()

The function mlxsw_sp_ipip_ol_netdev_change_gre4() contains code that
can be shared between IPv4 and IPv6.
The only difference is the way that arguments are taken from tunnel
parameters, which are different between IPv4 and IPv6.

For that, add structure 'mlxsw_sp_ipip_parms' to hold all the required
parameters for the function and save it as part of
'struct mlxsw_sp_ipip_entry' instead of the existing structure that is
not shared between IPv4 and IPv6. Add new operation as part of
'mlxsw_sp_ipip_ops' to initialize the new structure.

Then mlxsw_sp_ipip_ol_netdev_change_gre{4,6}() will prepare the new
structure and both will call the same function.

Signed-off-by: Amit Cohen <amcohen@nvidia.com>
Reviewed-by: Petr Machata <petrm@nvidia.com>
Signed-off-by: Ido Schimmel <idosch@nvidia.com>
Signed-off-by: David S. Miller <davem@davemloft.net>