[ATM]: No need to return void

The module_exit function has return-type void and pci_unregister_driver()
returns void anyway.

Signed-off-by: Tobias Klauser <tklauser@distanz.ch>
Signed-off-by: Andrew Morton <akpm@osdl.org>
Signed-off-by: David S. Miller <davem@davemloft.net>