net/mlx5: Warn when write combining is not supported

net/mlx5: Warn when write combining is not supported

Warn if write combining is not supported, as it can impact latency.
Add the warning message to be printed only when the driver actually
run the test and detect unsupported state, rather than when
inheriting parent's result for SFs.

Signed-off-by: Maor Gottlieb <maorg@nvidia.com>
Reviewed-by: Michael Guralnik <michaelgur@nvidia.com>
Signed-off-by: Tariq Toukan <tariqt@nvidia.com>
Reviewed-by: Simon Horman <horms@kernel.org>
Link: https://patch.msgid.link/1752009387-13300-5-git-send-email-tariqt@nvidia.com
Signed-off-by: Jakub Kicinski <kuba@kernel.org>