net/mlx5e: Move TX code into functions to be used by MPWQE

mlx5e_txwqe_complete performs some actions that can be taken to separate
functions:

1. Update the flags needed for hardware timestamping.

2. Stop the TX queue if it's full.

Take these actions into separate functions to be reused by the MPWQE
code in the following commit and to maintain clear responsibilities of
functions.

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