pcm: dmix: fix wrong scaling in 32bits pcm mixing

pcm: dmix: fix wrong scaling in 32bits pcm mixing

Generic mixing function for 32bits pcm has used 8bits right shift for
pre-scaling. But this is generating wrong result if pcm data is
negative value because return value type of bswap_32() is unsigned int.

This patch adds type cast bswap_32() result to signed int.

Fixes: https://github.com/alsa-project/alsa-lib/pull/222
Signed-off-by: Katsuhiro Suzuki <katsuhiro@katsuster.net>
Signed-off-by: Jaroslav Kysela <perex@perex.cz>