PCI: aerdrv: remove is_downstream

The pcie->port of port service device points the port associated
the service with.  The find_aer_service iterates over children of
given port udev.

So it is clear that the pcie->port of port service of given port
udev must always point the udev.

Therefore we can know the type of udev without checking its children.

Signed-off-by: Hidetoshi Seto <seto.hidetoshi@jp.fujitsu.com>
Reviewed-by: Kenji Kaneshige <kaneshige.kenji@jp.fujitsu.com>
Signed-off-by: Jesse Barnes <jbarnes@virtuousgeek.org>