lib/uuid.c: use correct offset in uuid parser

Use '+ 0' and '+ 1' as offsets, like they were intended, instead of
adding to the result.

Fixes: 2b1b0d66

 ("lib/uuid.c: introduce a few more generic helpers")
Signed-off-by: Bjørn Mork <bjorn@mork.no>
Signed-off-by: Andy Shevchenko <andriy.shevchenko@linux.intel.com>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>