USB: serial: qcserial: add DW5816e QDL support

Add support for Dell Wireless 5816e Download Mode (AKA boot & hold mode /
QDL download mode) to drivers/usb/serial/qcserial.c

This is required to update device firmware.

Signed-off-by: Matt Jolly <Kangie@footclan.ninja>
Cc: stable@vger.kernel.org
Signed-off-by: Johan Hovold <johan@kernel.org>