usb-mtp: fix sending files larger than 4gb

MTP requires that if a file is larger than 4gb or if sending data larger
than 4gb, that the length field be set to 0xFFFFFFFF.

Also widened a couple variables to prevent overflow errors.

Signed-off-by: Isaac Lozano <109lozanoi@gmail.com>
Message-id: 01ad8ec7775f58575801ac3f13716f553a16815e.1470477265.git.109lozanoi@gmail.com
Signed-off-by: Gerd Hoffmann <kraxel@redhat.com>
