net/mlx5e: Optimize XDP frame xmit

XDP SQ has a fixed size WQE (MLX5E_XDP_TX_WQEBBS = 1) and only posts
one kind of WQE (MLX5_OPCODE_SEND),

Also we initialize SQ descriptors static fields once on open_xdpsq,
rather than every time on critical path.

Optimize the code in light of those facts and add a prefetch of the TX
descriptor first thing in the xdp xmit function.

Performance improvement:
System: Intel(R) Xeon(R) CPU E5-2620 v3 @ 2.40GHz

Test case              Before     Now        improvement
---------------------------------------------------------------
XDP TX   (1 core)      13Mpps    13.7Mpps       5%

Signed-off-by: Saeed Mahameed <saeedm@mellanox.com>
Reviewed-by: Tariq Toukan <tariqt@mellanox.com>
Signed-off-by: David S. Miller <davem@davemloft.net>