net/mlx5: Fix read from coherent memory

Use accessor function READ_ONCE to read from coherent memory modified
by the device and read by the driver. This becomes most important in
preemptive kernels where cond_resched implementation does not have the
side effect which guaranteed the updated value.

Fixes: 269d26f4 ("net/mlx5: Reduce command polling interval")
Change-Id: Ie6deeb565ffaf76777b07448c7fbcce3510bbb8a
Signed-off-by: Eli Cohen <eli@mellanox.com>
Reported-by: Jesper Dangaard Brouer <brouer@redhat.com>
Signed-off-by: Saeed Mahameed <saeedm@mellanox.com>