net/mlx5e: Fix DCB_CAP_ATTR_DCBX capability for DCBNL getcap.

Current code doesn't report DCB_CAP_DCBX_HOST capability when query
through getcap. User space lldptool expects capability to have HOST mode
set when it wants to configure DCBX CEE mode. In absence of HOST mode
capability, lldptool fails to switch to CEE mode.

This fix returns DCB_CAP_DCBX_HOST capability when port's DCBX
controlled mode is under software control.

Fixes: 3a6a931d ("net/mlx5e: Support DCBX CEE API")
Signed-off-by: Huy Nguyen <huyn@mellanox.com>
Reviewed-by: Parav Pandit <parav@mellanox.com>
Signed-off-by: Saeed Mahameed <saeedm@mellanox.com>