media: technisat-usb2: break out of loop at end of buffer

Ensure we do not access the buffer beyond the end if no 0xff byte
is encountered.

Reported-by:  <syzbot+eaaaf38a95427be88f4b@syzkaller.appspotmail.com>
Signed-off-by: Sean Young <sean@mess.org>
Reviewed-by: Kees Cook <keescook@chromium.org>
Signed-off-by: Mauro Carvalho Chehab <mchehab+samsung@kernel.org>