libeffects: validate channel mask in downmixer configure

Validate channel mask in downmixer configure, this returns
early warning to calling module which can fallback to another
downmixing module or return error instead of playing improper
data.

authored-by: Dhananjay Kumar <dhakumar@codeaurora.org>

CRs-Fixed: 862105
Change-Id: Ic19c4913608468d5c5197f65bb5d6a640c9d2d74