tests: rtio: fix compile warning with arm-clang

When building the rtio_api tests with arm-clang we get the following
compiler warning:

rtio_api/src/test_rtio_api.c:436:58: warning: format specifies type
'unsigned long' but the argument has type 'uintptr_t'
(aka 'unsigned int') [-Wformat]

        TC_PRINT("userdata is %p, value %lu\n", cqe->userdata, idx);
                                        ~~~                    ^~~
                                        %u

Use PRIuPTR to fix the issue.

Signed-off-by: Kumar Gala <kumar.gala@intel.com>