dsp: codecs: fix range check for audio buffer copying

The range checking for audio buffer copying in function
"audio_in_write" is using the incorrect buffer size.
Change it to the actual allocated audio buffer size.

Change-Id: Icb9a9e9c3b3d3c6e55e7bad20b34f94784b82674
Signed-off-by: Xiaoyu Ye <benyxy@codeaurora.org>