exfat: Define NLS_NAME_* as bit flags explicitly

NLS_NAME_* are bit flags although they are currently defined as enum;
it's casually working so far (from 0 to 2), but it's error-prone and
may bring a problem when we want to add more flag.

This patch changes the definitions of NLS_NAME_* explicitly being bit
flags.

Reviewed-by: Petr Vorel <pvorel@suse.cz>
Signed-off-by: Takashi Iwai <tiwai@suse.de>
Signed-off-by: Namjae Jeon <linkinjeon@kernel.org>