After the last patch only three routines are left in this file, but all
of the fir into net/core/dev.c much better:

 - {un,}register_netdev are just wrappers around {un,}register_netdevice
   from dev.c
 - alloc_netdev's counterpart, free_netdev is in dev.c aswell.

So move over the remaining contents and add some kerneldoc comments
describing the functions.
Signed-off-by: David S. Miller <davem@davemloft.net>