exfat: improve error code handling in exfat_find_empty_entry()

exfat: improve error code handling in exfat_find_empty_entry()

Change the type of 'ret' from unsigned int to int in
exfat_find_empty_entry(). Although the implicit type conversion
(int -> unsigned int -> int) does not cause actual bugs in
practice, using int directly is more appropriate for storing
error codes returned by exfat_alloc_cluster().

This improves code clarity and consistency with standard error
handling practices.

Signed-off-by: Haotian Zhang <vulab@iscas.ac.cn>
Reviewed-by: Sungjong Seo <sj1557.seo@samsung.com>
Signed-off-by: Namjae Jeon <linkinjeon@kernel.org>