PCI/DPC: Add local variable for DPC capability offset

Add a local variable for DPC capability offset and replace repeated use of
"dpc->cap_pos" with simply "cap".  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>