net/mlx5: Fix misidentification of write combining CQE during poll loop

The write combining completion poll loop uses usleep_range() which can
sleep much longer than requested due to scheduler latency. Under load,
we witnessed a 20ms+ delay until the process was rescheduled, causing
the jiffies based timeout to expire while the thread is sleeping.

The original do-while loop structure (poll, sleep, check timeout) would
exit without a final poll when waking after timeout, missing a CQE that
arrived during sleep.

Instead of the open-coded while loop, use the kernel's poll_timeout_us()
which always performs an additional check after the sleep expiration,
and is less error-prone.

Note: poll_timeout_us() doesn't accept a sleep range, by passing 10
sleep_us the sleep range effectively changes from 2-10 to 3-10 usecs.

Fixes: d98995b4bf98 ("net/mlx5: Reimplement write combining test")
Signed-off-by: Gal Pressman <gal@nvidia.com>
Reviewed-by: Jianbo Liu <jianbol@nvidia.com>
Signed-off-by: Tariq Toukan <tariqt@nvidia.com>
Reviewed-by: Jacob Keller <Jacob.e.keller@intel.com>
Link: https://patch.msgid.link/20260218072904.1764634-4-tariqt@nvidia.com
Signed-off-by: Jakub Kicinski <kuba@kernel.org>

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