dcadec: treat all 2-channel modes as Stereo in dca_downmix.

The check for (prim_channels > 2) before calling dca_downmix made these
cases unreachable, but now 2.1 layouts will go through the downmix code.
Having dual mono, Lt/Rt and sum-difference layouts print errors when
regular Stereo doesn't seems pointless.

Signed-off-by: Anton Khirnov <anton@khirnov.net>