Bluetooth: Ignore hci_unregister_dev return value

Make all bluetooth drivers ignore the return value of hci_unregister_dev as it
always returns 0. In the next step, hci_unregister_dev can be modified to return
void.
Some of the drivers already ignore the return value (including btusb), hence,
this will increase consitency in the bluetooth drivers.

Signed-off-by: David Herrmann <dh.herrmann@googlemail.com>
Acked-by: Marcel Holtmann <marcel@holtmann.org>
Signed-off-by: Gustavo F. Padovan <padovan@profusion.mobi>