thunderbolt: Do not program NFC buffers for USB4 router protocol adapters

USB4 spec says that NFC buffers field is not used for protocol adapters,
only for lane adapters so make tb_port_add_nfc_credits() skip non-lane
adapters in order to follow the spec.

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