thunderbolt: Drop useless 'unidirectional' parameter from tb_switch_tmu_is_enabled()

There is no point passing it as we already have a field for that. While
there clean up the kernel-doc of things that do not really belong to the
API documentation (these can be figured out from the spec itself).

No functional changes.

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