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&gt;]>
Signed-off-by: Bjorn Helgaas <bhelgaas@google.com>