V4L/DVB (4711): Radio: 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: Mauro Carvalho Chehab <mchehab@infradead.org>