PCI: pciehp: Clear attention LED on device add

Clear the LED attention status after a successful device add.  It is
possible the attention LED was on from a previous power fault or link
failure, and a subsequent successful device insert insertion should clear
it.

Signed-off-by: Keith Busch <keith.busch@intel.com>
Signed-off-by: Bjorn Helgaas <bhelgaas@google.com>