lenovo/t520: fix PCIe interrupt and function disable config

Change-Id: I33e71c0a246583885368dc3d3af761c190b2fb5c
Signed-off-by: Nicolas Reinecke <nr@das-labor.org>
Reviewed-on: http://review.coreboot.org/6758
Tested-by: build bot (Jenkins)
Reviewed-by: Edward O'Callaghan <eocallaghan@alterapraxis.com>
Reviewed-by: Paul Menzel <paulepanter@users.sourceforge.net>