selftests/net: rxtimestamp: fix clang issues for target arch PowerPC

The signedness of char is implementation-dependent. Some systems
(including PowerPC and ARM) use unsigned char. Clang 9 threw:
warning: result of comparison of constant -1 with expression of type \
'char' is always true [-Wtautological-constant-out-of-range-compare]
                                  &arg_index)) != -1) {

Tested: make -C tools/testing/selftests TARGETS="net" run_tests

Fixes: 16e78122

 ("selftests/net: Add a test to validate behavior of rx timestamps")
Signed-off-by: Tanner Love <tannerlove@google.com>
Acked-by: Willem de Bruijn <willemb@google.com>
Signed-off-by: David S. Miller <davem@davemloft.net>