devlink: convert most of devlink_fmsg_*() to return void

Since struct devlink_fmsg retains error by now (see 1st patch of this
series), there is no longer need to keep returning it in each call.

This is a separate commit to allow per-driver conversion to stop using
those return values.

Reviewed-by: Jiri Pirko <jiri@nvidia.com>
Signed-off-by: Przemek Kitszel <przemyslaw.kitszel@intel.com>
Signed-off-by: David S. Miller <davem@davemloft.net>