gadget: f_thor: update to support more than 4GB file as thor 5.0

During file download, it only uses 32bit variable for file size and
it limits maximum file size less than 4GB. Update to support more
than 4GB file with using two 32bit variables for file size as thor
protocol 5.0.

Signed-off-by: Seung-Woo Kim <sw0312.kim@samsung.com>