PCI: endpoint: Add support to poll early for host commands

Certain platforms like TI's K2G doesn't support link-up notification. Add
support to poll early (without waiting for the linkup notification) for
commands from the host.

Signed-off-by: Kishon Vijay Abraham I <kishon@ti.com>
Signed-off-by: Bjorn Helgaas <bhelgaas@google.com>