asoc: adapt channel mixer usage to tinymix

Tinymix set the array value of the mixer control one by one. The
channel number validation check here will cause failure in tinymix.
Allow 0 value here is fine because the parameters will be checked
before they are set at runtime.

Change-Id: I49c23f84aa081550434696dde96d2800e90390fb
Signed-off-by: Erin Yan <xinyey@codeaurora.org>