mlxsw: spectrum: Prevent force of 56G

Force of 56G is not supported by hardware in Ethernet devices. This
configuration fails with a bad parameter error from firmware.

Add check of this case. Instead of trying to set 56G with autoneg off,
return a meaningful error.

Fixes: 56ade8fe

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