Casting 32bit int value directly into 64bit unsigned type causes
wrong value for file size equal or larger than 2GB. Fix the wrong
file size by casting uint32_t first.
Fixes: commit 1fe9ae76b113 ("gadget: f_thor: update to support more than 4GB file as thor 5.0") Reported-by: Junghoon Kim <jhoon20.kim@samsung.com> Signed-off-by: Seung-Woo Kim <sw0312.kim@samsung.com> Reviewed-by: Jaehoon Chung <jh80.chung@samsung.com>