net/mlx5e: Tx, no inline copy on ConnectX-5

ConnectX-5 and later HW generations will report min inline mode ==
MLX5_INLINE_MODE_NONE, which means driver is not required to copy packet
headers to inline fields of TX WQE.

When inline is not required, vlan insertion will be handled in the
TX descriptor rather than copy to inline.

For LSO case driver is still required to copy headers, for the HW to
duplicate on wire.

This will improve CPU utilization and boost TX performance.

Tested with pktgen burst single flow:
CPU: Intel(R) Xeon(R) CPU E5-2680 v3 @ 2.50GHz
HCA: Mellanox Technologies MT28800 Family [ConnectX-5 Ex]

Before: 15.1Mpps
After:  17.2Mpps
Improvement: 14%

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