netdevsim: declare struct device_type as static

struct device_type nsim_dev_type created for SR-IOV support
should be static.

Signed-off-by: Jakub Kicinski <jakub.kicinski@netronome.com>
Reviewed-by: Quentin Monnet <quentin.monnet@netronome.com>
Signed-off-by: David S. Miller <davem@davemloft.net>