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

[GitHub pull_request - edited]

alsa-project/alsa-lib pull request #222 was edited from katsuster:

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.

Request URL   : https://github.com/alsa-project/alsa-lib/pull/222
Patch URL     : https://github.com/alsa-project/alsa-lib/pull/222.patch
Repository URL: https://github.com/alsa-project/alsa-lib