net/mlx5e: Optimize poll ICOSQ completion queue

UMR operations are more frequent and important.
Check them first, and add a compiler branch predictor hint.

According to current design, ICOSQ CQ can contain at most one
pending CQE per napi. Poll function is optimized accordingly.

Performance:
Single-stream packet-rate tested with pktgen.
Packets are dropped in tc level to zoom into driver data-path.
Larger gain is expected for larger packet sizes, as BW is higher
and UMR posts are more frequent.

---------------------------------------------
packet size | before    | after     | gain  |
64B         | 4,092,370 | 4,113,306 |  0.5% |
1024B       | 3,421,435 | 3,633,819 |  6.2% |

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