vfat: deduplicate hex2bin()

We may use hex2bin() instead of custom approach.

Link: http://lkml.kernel.org/r/87zibktpil.fsf@devron


Signed-off-by: Andy Shevchenko <andriy.shevchenko@linux.intel.com>
Signed-off-by: OGAWA Hirofumi <hirofumi@mail.parknet.co.jp>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>