usb: ums - expose selected partition/s

By applying this patch, it will give us some flexibility to expose
a selected partition/s.

e.g:
1. To expose several partitions
ums 0 mmc 0:1,0:6

2. To expose the all partitions
ums 0 mmc 0:0

3. To expose multiple partititions on several devices
ums 0 mmc 0:1,1:6

4. It support legacy format
ums 0 mmc 0

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