net/mlx5e: Cleanup of start/stop all queues

mlx5e_activate_priv_channels() and mlx5e_deactivate_priv_channels()
start and stop all netdev TX queues. This commit removes the unneeded
call to netif_tx_stop_all_queues and adds explanatory comments why these
operations are needed.

netif_tx_disable() does the same thing that netif_tx_stop_all_queues(),
but taking the TX lock, thus guaranteeing that ndo_start_xmit is not
running after return. That means that the netif_tx_stop_all_queues()
call is not really necessary.

The comments are improved: the TX watchdog timeout explanation is moved
to the start stage where it really belongs (it used to be in both
places, but was lost during some old refactoring) and rephrased in more
details; the explanation for stopping all TX queues is added.

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