thunderbolt: Identify USB4 v2 routers

Add a new function usb4_switch_version() that can be used to figure out
the spec version of the router and make tb_switch_is_usb4() to use it as
well. Update the uevent accordingly.

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