net: devlink: add RNLT lock assertion to devlink_compat_switch_id_get()

Similar to devlink_compat_phys_port_name_get(), make sure that
devlink_compat_switch_id_get() is called with RTNL lock held. Comment
already says so, so put this in code as well.

Signed-off-by: Jiri Pirko <jiri@nvidia.com>
Link: https://lore.kernel.org/r/20220825112923.1359194-1-jiri@resnulli.us
Signed-off-by: Jakub Kicinski <kuba@kernel.org>