SoftOpus: Fix output buffer capacity.

The output buffer size as per opus project's sample decoder [1] is
960*6*channel_count. Whereas in SoftOpus, we use 960*6 (without the
channel count multiplier. Fixing it to include maximum number of
channels possible as the multiplier.

[1] http://git.xiph.org/?p=opus-tools.git;a=blob;f=src/opusdec.c;h=d085f04eacdfd49759ffdb73db805562ba396720;hb=f2a2e88b47f6f24083a37be476f140f677fe7160#l571

BUG=20721050

Change-Id: I323891a1b11491782bc093477b09e7757b885674