net/mlx5: Drain health before unregistering devlink

mlx5 health mechanism is using devlink APIs, which are using devlink
notify APIs. After the cited patch, using devlink notify APIs after
devlink is unregistered triggers a WARN_ON().
Hence, drain health WQ before devlink is unregistered.

Fixes: cf530217 ("devlink: Notify users when objects are accessible")
Signed-off-by: Shay Drory <shayd@nvidia.com>
Reviewed-by: Moshe Shemesh <moshe@nvidia.com>
Signed-off-by: Saeed Mahameed <saeedm@nvidia.com>