net/mlx5e: Set of completion request bit should not clear other adjacent bits

In notify HW (ring doorbell) flow, we set the bit to request a completion
on the TX descriptor.
When doing so, we should not unset other bits in the same byte.
Currently, this does not fix a real issue, as we still don't have a flow
where both MLX5_WQE_CTRL_CQ_UPDATE and any adjacent bit are set together.

Fixes: 542578c6 ("net/mlx5e: Move helper functions to a new txrx datapath header")
Fixes: 864b2d71

 ("net/mlx5e: Generalize tx helper functions for different SQ types")
Signed-off-by: Tariq Toukan <tariqt@mellanox.com>
Reviewed-by: Aya Levin <ayal@mellanox.com>
Signed-off-by: Saeed Mahameed <saeedm@mellanox.com>