dcadec: Do not decode the XCh extension when downmixing to stereo

This is neither necessary nor currently supported.

Signed-off-by: Luca Barbato <lu_zero@gentoo.org>
dcadec: Use correct channel count in stereo downmix check

s->prim_channels is greater than num_core_channels
when an XCh extension is present in the bitstream.

Signed-off-by: Luca Barbato <lu_zero@gentoo.org>