PCI/ASPM: Convert to use sysfs_match_string() helper

The sysfs_match_string() helper returns index of the matching string in an
array.  Use it in pcie_aspm_set_policy() to simplify the code.
Signed-off-by: Andy Shevchenko <andriy.shevchenko@linux.intel.com>
[bhelgaas: squash sysfs_match_string() fix into original patch for issue
Reported-by: Heiner Kallweit <hkallweit1@gmail.com>]
Signed-off-by: Bjorn Helgaas <bhelgaas@google.com>

drivers/pci/pcie/aspm.c

@@ -1127,11 +1127,9 @@ static int pcie_aspm_set_policy(const char *val,
 
 	if (aspm_disabled)
 		return -EPERM;
-	for (i = 0; i < ARRAY_SIZE(policy_str); i++)
-		if (!strncmp(val, policy_str[i], strlen(policy_str[i])))
-			break;
-	if (i >= ARRAY_SIZE(policy_str))
-		return -EINVAL;
+	i = sysfs_match_string(policy_str, val);
+	if (i < 0)
+		return i;
 	if (i == aspm_policy)
 		return 0;