Bluetooth: btusb: Refactor of firmware download flow for Intel conrollers

Address the scalability to support new generation Intel controller with
respect to readability and enhancement to new firmware download sequence

Signed-off-by: Kiran K <kiran.k@intel.com>
Reviewed-by: Chethan T N <chethan.tumkur.narayan@intel.com>
Signed-off-by: Marcel Holtmann <marcel@holtmann.org>