gadget: f_thor: fix wrong file size cast

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 1fe9ae76 ("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>