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>