cpufreq/amd-pstate: Make amd_pstate_get_mode_string() never return NULL

amd_pstate_get_mode_string() is only used by amd-pstate-ut.  Set the
failure path to use AMD_PSTATE_UNDEFINED ("undefined") to avoid showing
"(null)" as a string when running test suite.

Reviewed-by: Gautham R. Shenoy <gautham.shenoy@amd.com>
Signed-off-by: Mario Limonciello (AMD) <superm1@kernel.org>

Author: superm1

drivers/cpufreq/amd-pstate.c

@@ -70,8 +70,8 @@ static_assert(ARRAY_SIZE(amd_pstate_mode_string) == AMD_PSTATE_MAX);
 
 const char *amd_pstate_get_mode_string(enum amd_pstate_mode mode)
 {
-	if (mode < 0 || mode >= AMD_PSTATE_MAX)
-		return NULL;
+	if (mode < AMD_PSTATE_UNDEFINED || mode >= AMD_PSTATE_MAX)
+		mode = AMD_PSTATE_UNDEFINED;
 	return amd_pstate_mode_string[mode];
 }
 EXPORT_SYMBOL_GPL(amd_pstate_get_mode_string);