[NET]: unregister_netdevice as void

There was no real useful information from the unregister_netdevice() return
code, the only error occurred in a situation that was a driver bug. So
change it to a void function.

Signed-off-by: Stephen Hemminger <shemminger@osdl.org>
Signed-off-by: David S. Miller <davem@davemloft.net>