selftests: mlxsw: devlink_trap_tunnel_ipip: Send a full-length key

As part of adding same test for GRE tunnel with IPv6 underlay, missing
bytes for key were found.

mausezahn does not fill zeros between two colons, so send them
explicitly. For example, use "00:00:00:E9:" instead of ":E9:"

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