net/mlx5e: TX, Use actual WQE size for SQ edge fill

We fill SQ edge with NOPs to avoid WQEs wrap.
Here, instead of doing that in advance for the maximum possible
WQE size, we do it on-demand using the actual WQE size.
We re-order some parts in mlx5e_sq_xmit to finish the calculation
of WQE size (ds_cnt) before doing any writes to the WQE buffer.

When SQ work queue is fragmented (introduced in an downstream patch),
dealing with WQE wraps becomes more frequent. This change would drastically
reduce the overhead in this case.

Performance tests:
ConnectX-5 100Gbps, CPU: Intel(R) Xeon(R) CPU E5-2680 v3 @ 2.50GHz
Packet rate of 64B packets, single transmit ring, size 8K.

Before: 14.9 Mpps
After:  15.8 Mpps

Improvement of 6%.

Signed-off-by: Tariq Toukan <tariqt@mellanox.com>
Signed-off-by: Saeed Mahameed <saeedm@mellanox.com>