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.