PCI/PTM: Cache PTM Capability offset

Cache the PTM Capability offset instead of searching for it every time we
enable/disable PTM or save/restore PTM state.  No functional change
intended.

Link: https://lore.kernel.org/r/20220909202505.314195-2-helgaas@kernel.org
Tested-by: Rajvi Jingar <rajvi.jingar@linux.intel.com>
Signed-off-by: Bjorn Helgaas <bhelgaas@google.com>
Reviewed-by: Kuppuswamy Sathyanarayanan <sathyanarayanan.kuppuswamy@linux.intel.com>
Reviewed-by: Mika Westerberg <mika.westerberg@linux.intel.com>