net: devlink: use helpers to work with devlink->lock mutex

As far as the lock helpers exist as the drivers need to work with the
devlink->lock mutex, use the helpers internally in devlink.c in order to
be consistent.
Signed-off-by: Jiri Pirko <jiri@nvidia.com>
Acked-by: Jakub Kicinski <kuba@kernel.org>
Signed-off-by: David S. Miller <davem@davemloft.net>