IB/mlx5: Call io_stop_wc() after writing to WC MMIO

This new function is defined only on ARM and serves to guarantee a barrier
in the WC operation. The barrier means that another run of this loop will
not combine with the stores this loop created.

On x86 this is happening implicitly because of the spin_unlock().

Link: https://lore.kernel.org/r/0-v1-c5dade92f363+11-mlx5_io_stop_wc_jgg@nvidia.com


Suggested-by: Pavel Shamis <Pavel.Shamis@arm.com>
Signed-off-by: Jason Gunthorpe <jgg@nvidia.com>
Signed-off-by: Leon Romanovsky <leon@kernel.org>