USB: g_dnl: Change device class

The USB Mass Storage (ums) works in Windows, Linux and OS X (EL Capitan).
But, not in OS X (Yosemite). By applying the said patch, it extends
the ums support.

Signed-off-by: John Tobias <john.tobias.ph@gmail.com>
Tested-by: Lukasz Majewski <l.majewski@samsung.com>

Test HW: Odroid XU3 (./test/py UMS + DFU tests)

Tested-by: John Tobias <john.tobias.ph@gmail.com>

Linux:
    - Run ums to expose all my eMMC partition - shows all correctly
    - Run ums to expose only 1 partition of my eMMC - show correctly

Windows:
    - Run ums to expose all my eMMC partition - it detects but it prompts,
    if I want to format it (due to a non windows partition)
    - Run ums to expose only the FAT32 partition - it show the partition
      correctly.