We already map the in:out 1:1 mappings above the switch statement, but
the code for "simple" mapping stomps on the mappings.

Also, even before that loop was added, mix mode "simple" did not
necessarily map to the same channel types you'd expect. For example, if
you have a 6-channel input and 8-channel output, you would get something
like this with "simple":

  Correct:  CHANNEL_FRONT_LEFT -> CHANNEL_FRONT_LEFT
  Correct:  CHANNEL_FRONT_RIGHT -> CHANNEL_FRONT_RIGHT
  Correct:  CHANNEL_FRONT_CENTER -> CHANNEL_FRONT_CENTER
  Correct:  CHANNEL_LFE -> CHANNEL_LFE
  Wrong:    CHANNEL_SIDE_LEFT -> CHANNEL_BACK_LEFT
  Wrong:    CHANNEL_SIDE_RIGHT -> CHANNEL_BACK_RIGHT

Instead of just assuming the input channel map is a truncation of the
output channel map and has channels in the same order, just keep the
existing 1:1 mappings and hope that's sufficient.
Signed-off-by: Steven Noonan <steven@uplinklabs.net>