devlink: Split out dev get and dump code

Move devlink dev get and dump callbacks and related dev code to new file
dev.c. This file shall include all callbacks that are specific on
devlink dev object.

No functional change in this patch.

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