net/mlx5e: Check for qos capability in dcbnl_initialize

qos capability is the master capability bit that determines
if the DCBX is supported for the PCI function. If this bit is off,
driver cannot run any dcbx code.

Fixes: e207b7e9 ("net/mlx5e: ConnectX-4 firmware support for DCBX")
Signed-off-by: Huy Nguyen <huyn@mellanox.com>
Reviewed-by: Parav Pandit <parav@mellanox.com>
Signed-off-by: Saeed Mahameed <saeedm@mellanox.com>