PCI/DPC: Fix interrupt message number print

The interrupt message number is the first 5 bits, but the driver was
masking only the first 4 bits.  Fix that by using the existing
define.

Signed-off-by: Keith Busch <keith.busch@intel.com>
[bhelgaas: remove reformatting (done by another patch)]
Signed-off-by: Bjorn Helgaas <bhelgaas@google.com>