selftests: mlxsw: Check devlink device before running test

The scale test for Spectrum-2 should only be invoked for Spectrum-2.
Skip the test otherwise.

Signed-off-by: Danielle Ratson <danieller@mellanox.com>
Signed-off-by: Ido Schimmel <idosch@mellanox.com>
Signed-off-by: David S. Miller <davem@davemloft.net>