Bluetooth: btusb: add sysfs attribute to control USB alt setting

When a Bluetooth raw socket is open, the HCI event related to SCO
connection changes are not dispatched to the hci_event module, and
the underlying Bluetooth controller's USB Interface 1 will not be
updated accordingly.

This patch adds `isoc_alt` sysfs attribute, allowing user space
to update the alternate setting of the USB interface alternate
setting as needed.

Signed-off-by: Ying Hsu <yinghsu@chromium.org>
Signed-off-by: Luiz Augusto von Dentz <luiz.von.dentz@intel.com>