netdevsim: Make nsim_num_vf static

Fix sparse warning:

drivers/net/netdevsim/bus.c:253:5: warning:
 symbol 'nsim_num_vf' was not declared. Should it be static?

Reported-by: Hulk Robot <hulkci@huawei.com>
Signed-off-by: YueHaibing <yuehaibing@huawei.com>
Acked-by: Jakub Kicinski <jakub.kicinski@netronome.com>
Signed-off-by: David S. Miller <davem@davemloft.net>