gadget: f_thor: fix filename overflow

The thor sender can send filename without null character and it is
used without consideration of overflow. Actually, character array
for filename is assigned with DEFINE_CACHE_ALIGN_BUFFER() and it
is bigger than size of memcpy, so there was no real overflow.
Fix filename overflow for code level integrity.

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