[Bluetooth] Add SCO support to btusb driver

The new generic driver for Bluetooth USB devices was missing proper
SCO support. The driver now claims the second interface for these USB
devices to allow the flow of SCO packets. It also handles switching
of the alternate setting and re-submission of isochronous URBs.

The btusb driver is now a full replacement for hci_usb and thus the
experimental tag has been removed and this driver is promoted as
preferred one.

Signed-off-by: Marcel Holtmann <marcel@holtmann.org>