target/nds32_disassembler: fix format specifiers warnings

According to the standard every operation returns at least an integer,
so PRIu8 format specifier is not suitable for these values as is.

This breaks build on OS X (x86_64-apple-darwin13.0.0) with "Apple LLVM
version 5.0 (clang-500.2.79) (based on LLVM 3.3svn)".

Fix by adding appropriate casts. In fact there's plenty of room (and
I'd say necessity) for factoring out common code in there, but it's
too invasive for a non-maintainer.

Change-Id: I7d2182eb1d2f86fa22c882fbbaa6cfadf1c3e8fc
Signed-off-by: Paul Fertser <fercerpav@gmail.com>
Reviewed-on: http://openocd.zylin.com/1878


Tested-by: jenkins
Reviewed-by: Xiaofan <xiaofanc@gmail.com>
Reviewed-by: Hsiangkai Wang <hsiangkai@gmail.com>
Reviewed-by: Spencer Oliver <spen@spen-soft.co.uk>