USB: serial: qcserial: add Dell DW5570 QDL

The Dell DW5570 is a re-branded Sierra Wireless MC8805 which will by
default boot with vid 0x413c and pid 0x81a3. When triggered QDL download
mode, the device switches to pid 0x81a6 and provides the standard TTY
used for firmware upgrade.

Cc: <stable@vger.kernel.org>
Signed-off-by: Aleksander Morgado <aleksander@aleksander.es>
Signed-off-by: Johan Hovold <johan@kernel.org>