thunderbolt: Check for first depth router in tb.c

Currently tb_switch_clx_enable() enables CL states only for the first
depth router. This is something we may want to change in the future and
in addition it is not visible from the calling path at all. For this
reason do the check in the tb.c so it is immediately visible that we
only do this for the first depth router. Fix the kernel-docs
accordingly.

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