devlink: Move health common function to health file

Now that all devlink health callbacks and related code are in file
health.c move common health functions and devlink_health_reporter struct
to be local in health.c file.

Signed-off-by: Moshe Shemesh <moshe@nvidia.com>
Reviewed-by: Jiri Pirko <jiri@nvidia.com>
Reviewed-by: Jakub Kicinski <kuba@kernel.org>
Signed-off-by: Jakub Kicinski <kuba@kernel.org>