mlx4_core: Add module parameter to enable QoS support

Add a module parameter "enable_qos" to mlx4_core.  If this param is
set, enable support for QoS in the INIT_HCA command.  By default, the
parameter is set to 0 (disabled).

Signed-off-by: Jack Morgenstein <jackm@dev.mellanox.co.il>
Signed-off-by: Roland Dreier <rolandd@cisco.com>