PCI/P2PDMA: Use pci_dev_id() to simplify the code

When we have a struct pci_dev *, use pci_dev_id() instead of manually
composing the ID from dev->bus->number and dev->devfn.

[bhelgaas: commit log]
Link: https://lore.kernel.org/r/20230811111057.31900-1-zhengzengkai@huawei.com


Signed-off-by: Zheng Zengkai <zhengzengkai@huawei.com>
Signed-off-by: Bjorn Helgaas <bhelgaas@google.com>
Reviewed-by: Logan Gunthorpe <logang@deltatee.com>