ex: Fix mismatch between header and implementation

The header defines GetChannelCount() as returning a size_t while
the implementation defines it as returning a SLuint32.

Change-Id: Iaa837d9a39540c338582496a33a4bbd8e3c63b55
Signed-off-by: Bernhard Rosenkraenzer <Bernhard.Rosenkranzer@linaro.org>