USB: qcserial: add Sierra Wireless MC74xx/EM74xx

New device IDs shamelessly lifted from the vendor driver.

Cc: <stable@vger.kernel.org>
Signed-off-by: Bjørn Mork <bjorn@mork.no>
Acked-by: Johan Hovold <johan@kernel.org>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>