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 42a7feda64a7 ("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 <jh80.chung@samsung.com>
Signed-off-by: Seung-Woo Kim <sw0312.kim@samsung.com>
Reviewed-by: Lukasz Majewski <lukma@denx.de>