PCI/AER: Simplify __aer_print_error()

aer_correctable_error_string[] and aer_uncorrectable_error_string[] have
descriptions of AER error status bits.  Add NULL entries to these tables so
all entries for bits 0-31 are defined.  Then we don't have to check for
ARRAY_SIZE() when decoding a status word, which simplifies
__aer_print_error().

Signed-off-by: Bjorn Helgaas <bhelgaas@google.com>