mlxsw: spectrum_ipip: Pass IP tunnel parameters by reference and as 'const'

Currently, there are several functions that handle 'struct ip_tunnel_parm'
and 'struct __ip6_tnl_parm'.

Change the mentioned functions to get IP tunnel parameters by reference
and as 'const'.

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>