PCI/P2PDMA: Use for_each_pci_dev() helper

Use for_each_pci_dev() instead of open-coding it.  No functional change.

Link: https://lore.kernel.org/r/20220916140329.679633-1-yangyingliang@huawei.com


Signed-off-by: Yang Yingliang <yangyingliang@huawei.com>
Signed-off-by: Bjorn Helgaas <bhelgaas@google.com>
Reviewed-by: Logan Gunthorpe <logang@deltatee.com>