bluetooth: Make hci_sock_cleanup() return void

hci_sock_cleanup() always returns 0 and its return value isn't used
anywhere in the code.

Compile-tested with 'make allyesconfig && make net/bluetooth/bluetooth.ko'

Signed-off-by: Tobias Klauser <tklauser@distanz.ch>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Acked-by: Marcel Holtmann <marcel@holtmann.org>