PCI/AER: Use match_string() helper to simplify the code

match_string() returns the array index of a matching string.  Use it
instead of the open-coded implementation.

Signed-off-by: Andy Shevchenko <andriy.shevchenko@linux.intel.com>
Signed-off-by: Bjorn Helgaas <bhelgaas@google.com>