fs: fat: fix wrong casting to unsigned value of sect_to_cluster()

fs: fat: fix wrong casting to unsigned value of sect_to_cluster()

After the commit 265edc03d5a1 ("fs/fat: Clean up open-coded sector
<-> cluster conversions"), it is hung up writing new file to FAT16
disk with more than 19 files in armv7. It is because result value
of sect_to_cluster() is not proper by casting from signed value to
unsigned value. Fix the wrong casting of sect_to_cluster().

Reported-by: Jaehoon Chung <[email protected]>
Signed-off-by: Seung-Woo Kim <[email protected]>
Reviewed-by: Lukasz Majewski <[email protected]>