PCI/portdrv: Add #defines for AER and DPC Interrupt Message Number masks

In the AER case, the mask isn't strictly necessary because there are no
higher-order bits above the Interrupt Message Number, but using a #define
will make it possible to grep for it.

Suggested-by: Bjorn Helgaas <bhelgaas@google.com>
Signed-off-by: Dongdong Liu <liudongdong3@huawei.com>
Signed-off-by: Bjorn Helgaas <bhelgaas@google.com>
Reviewed-by: Christoph Hellwig <hch@lst.de>