thunderbolt: Move nfc_credits field to struct tb_path_hop

With the USB4 buffer allocation the number of credits (and non-flow
credits) may be different depending on the router buffer allocation
preferences. To allow this move the nfc_credits field to struct
tb_path_hop.

Signed-off-by: Mika Westerberg <mika.westerberg@linux.intel.com>
Change-Id: I1191f10dd4cc0b71ae73e800ee8afd625c677e9b