thunderbolt: There are only 5 basic router registers in pre-USB4 routers

Intel pre-USB4 routers only have ROUTER_CS_0 up to ROUTER_CS_4 and it
immediately follows the TMU router registers. Correct this accordingly.

Reported-by: Rajaram Regupathy <rajaram.regupathy@intel.com>
Signed-off-by: Mika Westerberg <mika.westerberg@linux.intel.com>