exfat: remove the assignment of 0 to bool variable

There is no need to init 'sync' in exfat_set_vol_flags().
This also fixes the following coccicheck warning:

fs/exfat/super.c:104:6-10: WARNING: Assignment of 0/1 to bool variable

Signed-off-by: Jason Yan <yanaijie@huawei.com>
Signed-off-by: Namjae Jeon <namjae.jeon@samsung.com>