[NET]: Fix wrong comments for unregister_net*

There are some return value comments for void functions.
Fixed it.

Signed-off-by: Wang Chen <wangchen@cn.fujitsu.com>
Signed-off-by: David S. Miller <davem@davemloft.net>

net/core/dev.c

@@ -3972,8 +3972,7 @@ void synchronize_net(void)
  *	@dev: device
  *
  *	This function shuts down a device interface and removes it
- *	from the kernel tables. On success 0 is returned, on a failure
- *	a negative errno code is returned.
+ *	from the kernel tables.
  *
  *	Callers must hold the rtnl semaphore.  You may want
  *	unregister_netdev() instead of this.
@@ -3991,8 +3990,7 @@ void unregister_netdevice(struct net_device *dev)
  *	@dev: device
  *
  *	This function shuts down a device interface and removes it
- *	from the kernel tables. On success 0 is returned, on a failure
- *	a negative errno code is returned.
+ *	from the kernel tables.
  *
  *	This is just a wrapper for unregister_netdevice that takes
  *	the rtnl semaphore.  In general you want to use this and not