[mtp] ignore storage ID sent in MTP object info

Some initiators leave this field zero. We should always take into
account only the storage ID parameter of MTP request. Not doing so
resulted in files ending up on the first storage (since ID 0 means
responder should choose), and if parent object was also specified, error
response with InvalidParentObject was generated.
Merge pull request #46 from xhaakon/update

[mtp] ignore storage ID sent in MTP object info