PCI/DPC: Add local struct device pointers

Use a local "struct device *dev" for brevity and consistency in DPC driver.
No functional change intended.

Signed-off-by: Dongdong Liu <liudongdong3@huawei.com>
Signed-off-by: Bjorn Helgaas <bhelgaas@google.com>
Reviewed-by: Keith Busch <keith.busch@intel.com>