PCI/ASPM: Use correct capability pointer to program LTR_L1.2_THRESHOLD

Previously we programmed the LTR_L1.2_THRESHOLD in the parent (upstream)
device using the capability pointer of the *child* (downstream) device,
which corrupted some random word of the parent's config space.

Use the parent's L1 SS capability pointer to program its
LTR_L1.2_THRESHOLD.

Fixes: aeda9ade ("PCI/ASPM: Configure L1 substate settings")
Signed-off-by: Bjorn Helgaas <bhelgaas@google.com>
Reviewed-by: Vidya Sagar <vidyas@nvidia.com>
CC: stable@vger.kernel.org	# v4.11+
CC: Rajat Jain <rajatja@google.com>