mlxsw: spectrum: Indicate support for autonegotiation

The device supports link autonegotiation, so let the user know about it
by indicating support via ethtool ops.

Fixes: 56ade8fe

 ("mlxsw: spectrum: Add initial support for Spectrum ASIC")
Signed-off-by: Ido Schimmel <idosch@mellanox.com>
Signed-off-by: Jiri Pirko <jiri@mellanox.com>
Signed-off-by: David S. Miller <davem@davemloft.net>