hal: do not set custom channel map for mono and stereo clips

In case of compress playback for some mono clips, DSP assumes
the channel map to be FL/FR, while default custom channel map
is set to FC, then volume can't be adjusted.
For mono and stereo clips, don't set custom channel map if
it's not specified by clients. This can ensure volume can
always be applied for FL/FR/FC channels.

Change-Id: I28aea5cf98a0a5d794669bc1a8fce674abbd18eb