net: Do not return value from init_dummy_netdev()

init_dummy_netdev() always returns zero and all the callers do not check
the returned value. Set the function to not return value, as it is not
really used today.

Signed-off-by: Amit Cohen <amcohen@nvidia.com>
Reviewed-by: Ido Schimmel <idosch@nvidia.com>
Reviewed-by: Jiri Pirko <jiri@nvidia.com>
Reviewed-by: Simon Horman <horms@kernel.org>
Link: https://lore.kernel.org/r/20240205103022.440946-1-amcohen@nvidia.com


Signed-off-by: Jakub Kicinski <kuba@kernel.org>