net/mlx5e: Maintain MQPRIO mode parameter

This is in preparation for supporting MQPRIO CHANNEL mode in
downstream patch, in addition to DCB mode that's supported today.

Signed-off-by: Tariq Toukan <tariqt@nvidia.com>
Reviewed-by: Maxim Mikityanskiy <maximmi@nvidia.com>
Signed-off-by: Saeed Mahameed <saeedm@nvidia.com>