exfat: fix range validation error in alloc and free cluster

There is check error in range condition that can never be entered
even with invalid input.
Replace incorrent checking code with already existing valid checker.

Signed-off-by: hyeongseok.kim <hyeongseok@gmail.com>
Acked-by: Sungjong Seo <sj1557.seo@samsung.com>
Signed-off-by: Namjae Jeon <namjae.jeon@samsung.com>