Fix generation of IEC958 subframes
From: Nicolas Boullis <nboullis@debian.org>
While working on an ALSA driver for the audio part of em8300-based MPEG
decoder boards (Sigma Designs' Hollywood+ boards and Creative Labs' DXR3
boards), I think I discovered a bug in the code that generates IEC958
subframes from linear samples (iec958 plugin): M and W preambles are
swapped, and B preambles should only be used on even subframes. The
attached patch fixes this issue.