USB: qcserial: Enable Diagnostics Monitor and GPS ports on Gobi 2000

this patch to qcserial.c enables the Diagnostics Monitor
and NMEA GPS ports on Qualcomm Gobi 2000 devices.

A Gobi 2000 device will provide 3 serial ports:
        # /dev/ttyUSB0 -> Diagnostics
        # /dev/ttyUSB1 -> 3G Modem
        # /dev/ttyUSB2 -> NMEA GPS port

* The Diagnostics Monitor uses Qualcomm's DM protocol; I used
  libqcdm (ModemManager) to talk to it, found it working, but at
  least DM commands 12 and 64 are not implemented on my device
  (Gobi 2000 built into Thinkpad x100e).

* Functionality of the 3G Modem port remains unchanged.

* The GPS port and how to enable it has been confirmed now in the
  Gobi 3000 source code at:
	https://www.codeaurora.org/patches/quic/gobi/


  Enable/disable GPS via:
        echo "\$GPS_START" > /dev/ttyUSB2
        # use GPS
        echo "\$GPS_STOP"  > /dev/ttyUSB2


Signed-off-by: Matthias G. Eckermann <mge@arcor.de>