ASoC: msmcobalt: fix proxy channels index value

The array index value when setting/getting the channels
of proxy is incorrect as the channel count begins from two.
Fix by setting the correct index offset.

Change-Id: I3ab9858daf6dcdbce762946cbdebeb96eaad764a
Signed-off-by: Shiv Maliyappanahalli <smaliyap@codeaurora.org>