audio: fix integer overflow expression

Fix an integer overflow that can happen for signed 32 bit types
when using FLOAT_MIXENG. (Note that at the moment this is only true
when using the MacOSX coreaudio audio driver.)

Signed-off-by: Juha Riihim?ki <juha.riihimaki@nokia.com>
[Peter Maydell: Removed unnecessary casts]
Signed-off-by: Peter Maydell <peter.maydell@linaro.org>
Signed-off-by: malc <av1474@comtv.ru>
