net/mlx5: Reduce command polling interval

Use cond_resched() instead of usleep_range() to decrease the time
between polling attempts thus reducing overall driver load time.

Below is a comparison before and after the change, of loading eight
virtual functions.

Before:
real    0m8.785s
user    0m0.093s
sys     0m0.090s

After:
real    0m5.730s
user    0m0.097s
sys     0m0.087s

Signed-off-by: Eli Cohen <eli@mellanox.com>
Reviewed-by: Or Gerlitz <ogerlitz@mellanox.com>
Signed-off-by: Saeed Mahameed <saeedm@mellanox.com>
Signed-off-by: David S. Miller <davem@davemloft.net>

drivers/net/ethernet/mellanox/mlx5/core/cmd.c

@@ -211,7 +211,7 @@ static void poll_timeout(struct mlx5_cmd_work_ent *ent)
 			ent->ret = 0;
 			return;
 		}
-		usleep_range(5000, 10000);
+		cond_resched();
 	} while (time_before(jiffies, poll_end));
 
 	ent->ret = -ETIMEDOUT;