net/mlx5e: Enable CQE based moderation on TX CQ

By using CQE based moderation on TX CQ we can reduce the number of TX
interrupt rate. Besides the benefit of less interrupts, this also
allows the kernel to better utilize TSO. Since TSO has some CPU overhead,
it might not aggregate when CPU is under high stress. By reducing the
interrupt rate and the CPU utilization, we can get better aggregation
and better overall throughput.
The feature is enabled by default and has a private flag in ethtool
for control.

Throughput, interrupt rate and TSO utilization improvements:
(ConnectX-4Lx 40GbE, unidirectional, 1/16 TCP streams, 64B packets)
---------------------------------------------------------
Metric   | Streams | CQE Based | EQE Based | improvement
---------------------------------------------------------
BW       |    1    |  2.4Gb/s  | 2.15Gb/s  |  +11.6%
IR       |    1    |  27Kips   | 50.6Kips  |  -46.7%
TSO Util |    1    |  74.6%    | 71%       |  +5%
BW       |    16   |  29Gb/s   | 25.85Gb/s |  +12.2%
IR       |    16   |  482Kips  | 745Kips   |  -35.3%
TSO Util |    16   |  69.1%    | 49%       |  +41.1%

*BW = Bandwidth, IR = Interrupt rate, ips = interrupt per second.
TSO Util = bytes in TSO sessions / all bytes transferred

Signed-off-by: Tal Gilboa <talgi@mellanox.com>
Signed-off-by: Saeed Mahameed <saeedm@mellanox.com>