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: NHulk Robot <hulkci@huawei.com>
Signed-off-by: NYueHaibing <yuehaibing@huawei.com>
Acked-by: NJakub Kicinski <jakub.kicinski@netronome.com>
Signed-off-by: NDavid S. Miller <davem@davemloft.net>

drivers/net/netdevsim/bus.c

@@ -250,7 +250,7 @@ static int nsim_bus_remove(struct device *dev)
 	return 0;
 }
 
-int nsim_num_vf(struct device *dev)
+static int nsim_num_vf(struct device *dev)
 {
 	struct nsim_bus_dev *nsim_bus_dev = to_nsim_bus_dev(dev);