PCI: endpoint: Add an API to get matching "pci_epf_device_id"

Add an API to get "pci_epf_device_id" matching the EPF name. This can be
used by the EPF driver to get the driver data corresponding to the EPF
device name.

Signed-off-by: Kishon Vijay Abraham I <kishon@ti.com>
[bhelgaas: folded in "while" loop termination fix from Colin Ian King
<colin.king@canonical.com>]
Signed-off-by: Bjorn Helgaas <bhelgaas@google.com>