According to the AB8500 user manual AD to Slot register multiplexer
accept values from 0 to 15 where:

   0 to  7 corresponds to AD_OUTx slots
   8 to 11 corresponds to zero output
  12 to 15 sets the output in tristate mode

Update enum_ad_to_slot_map array to reflect this definition.

This also allows alsamixer to properly display the default
configuration, as all controls are set to tristate (=12) at reset.
Signed-off-by: Fabio Baltieri <fabio.baltieri@linaro.org>
Acked-by: Lee Jones <lee.jones@linaro.org>
Signed-off-by: Mark Brown <broonie@opensource.wolfsonmicro.com>