PCI: aerdrv: rework aer_isr_one_error()

Divide tricky for-loop into readable if-blocks.

The logic to set multi_error_valid (to force walking pci bus
hierarchy to find 2nd~ error devices) is changed too, to check
MULTI_{,_UN}COR_RCV bit individually and to force walk only when
it is required.

And rework setting e_info->severity for uncorrectable, not to use
magic numbers.

Signed-off-by: Hidetoshi Seto <seto.hidetoshi@jp.fujitsu.com>
Reviewed-by: Kenji Kaneshige <kaneshige.kenji@jp.fujitsu.com>
Signed-off-by: Jesse Barnes <jbarnes@virtuousgeek.org>