thunderbolt: Read router preferred credit allocation information

USB4 routers must expose their preferred credit (buffer) allocation
information through router operation. This information tells the
connection manager how the router prefers its buffers to be allocated to
get the expected bandwidth for the supported protocols.

Read this information and store it as part of struct tb_switch for each
USB4 router.

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