net: wwan: iosm: Move devlink_register to be last devlink command

This change prevents from users to access device before devlink is
fully configured. Indirectly this change fixes the commit mentioned
below where devlink_unregister() was prematurely removed.

Fixes: db4278c5 ("devlink: Make devlink_register to be void")
Signed-off-by: Leon Romanovsky <leonro@nvidia.com>
Signed-off-by: David S. Miller <davem@davemloft.net>