thunderbolt: Correct TMU mode initialization from hardware

David reported that cppcheck found following possible copy & paste
error from tmu_mode_init():

  tmu.c:385:50: style: Expression is always false because 'else if' condition matches previous condition at line 383. [multiCondition]

And indeed this is a bug. Fix it to use correct index
(TB_SWITCH_TMU_MODE_HIFI_UNI).

Reported-by: David Binderman <dcb314@hotmail.com>
Fixes: d49b4f04

 ("thunderbolt: Add support for enhanced uni-directional TMU mode")
Cc: stable@vger.kernel.org
Signed-off-by: Mika Westerberg <mika.westerberg@linux.intel.com>