selftests/powerpc/pmu: Fix unsigned function returning negative constant

The function __perf_reg_mask has an unsigned return type, but returns a
negative constant to indicate an error condition. So we change unsigned
to int.
Signed-off-by: Haowen Bai <baihaowen@meizu.com>
Signed-off-by: Michael Ellerman <mpe@ellerman.id.au>
Link: https://lore.kernel.org/r/1650788802-14402-1-git-send-email-baihaowen@meizu.com

tools/testing/selftests/powerpc/pmu/sampling_tests/misc.c

@@ -274,7 +274,7 @@ u64 *get_intr_regs(struct event *event, void *sample_buff)
 	return intr_regs;
 }
 
-static const unsigned int __perf_reg_mask(const char *register_name)
+static const int __perf_reg_mask(const char *register_name)
 {
 	if (!strcmp(register_name, "R0"))
 		return 0;