netdevsim: Change nsim_devlink_setup to return error to caller

Change nsim_devlink_setup to return any error back to the caller and
update nsim_init to handle it.

Requested-by: Jakub Kicinski <jakub.kicinski@netronome.com>
Signed-off-by: David Ahern <dsa@cumulusnetworks.com>
Acked-by: Jakub Kicinski <jakub.kicinski@netronome.com>
Signed-off-by: David S. Miller <davem@davemloft.net>