mlxsw: spectrum: Correctly report autonegotiation

Up until now the device always reported autonegotiation to be off
although it was on by default.

Allow the user to disable / enable autonegotiation and report its status
correctly.

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>