Bluetooth: btusb: Consolidate code for waiting firmware to boot

This moves duplicated code for waiting firmware download completion to
a function that can be reused.

Signed-off-by: Luiz Augusto von Dentz <luiz.von.dentz@intel.com>
Tested-by: Tedd Ho-Jeong An <tedd.an@intel.com>
Signed-off-by: Marcel Holtmann <marcel@holtmann.org>