Now this is split into two with one having a misleading name
(tb_switch_tmu_unidirectional_enable()).

Make this easier to read, rename and consolidate the two functions into
one with name that explains what it actually does. Use the two constants
as well that were added but never used to make it clear which bits are
being set.

No functional changes.
Signed-off-by: Mika Westerberg <mika.westerberg@linux.intel.com>