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>
(cherry picked from commit c8cf461c19e8e35df4b7364d9b90aa42f1ab4560)
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>
(cherry picked from commit 801c39e1e3058fc4ba822bfb5d8612d777111e32)