PCI/DPC: Reformat DPC register definitions

Reformat DPC register definitions to follow the convention that register
field masks indicate the register width, e.g., a field of a 16-bit register
uses a mask of 4 hex digits, with leading zeros included as needed.
No functional change intended.

Signed-off-by: Bjorn Helgaas <bhelgaas@google.com>
Reviewed-by: Keith Busch <keith.busch@intel.com>
Reviewed-by: Sinan Kaya <okaya@codeaurora.org>