exfat: Simplify exfat_utf8_d_hash() for code points above U+FFFF

Function partial_name_hash() takes long type value into which can be stored
one Unicode code point. Therefore conversion from UTF-32 to UTF-16 is not
needed.

Signed-off-by: Pali Rohár <pali@kernel.org>
Signed-off-by: Namjae Jeon <namjae.jeon@samsung.com>