netdevsim: add switch_id attribute

Grouping netdevsim devices into "ASICs" will soon be supported.
Add switch_id attribute to all netdevsims.  For now each netdevsim
will have its switch_id matching the device id.

Signed-off-by: Jakub Kicinski <jakub.kicinski@netronome.com>
Reviewed-by: Quentin Monnet <quentin.monnet@netronome.com>
Signed-off-by: Daniel Borkmann <daniel@iogearbox.net>