thunderbolt: Use correct function to calculate maximum USB3 link rate

We need to take minimum of both sides of the USB3 link into consideration,
not just the downstream port. Fix this by calling tb_usb3_max_link_rate()
instead.

Fixes: 0bd680cd

 ("thunderbolt: Add USB3 bandwidth management")
Cc: stable@vger.kernel.org
Acked-by: Yehezkel Bernat <YehezkelShB@gmail.com>
Signed-off-by: Mika Westerberg <mika.westerberg@linux.intel.com>