net/mlx5e: Poll XDP TX CQ before RX CQ

Handle XDP TX completions before handling RX packets, to make sure more
free space is available for XDP TX packets a moment before handling
RX packets.

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

Test case              Before     Now      improvement
---------------------------------------------------------------
XDP Drop (1 core)      16.9Mpps  16.9Mpps    No change
XDP TX   (1 core)      12Mpps    13Mpps      8%

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