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: Ib7aaa2163c0d99161369eb85d09dc2d23d8c787b
Signed-off-by: Xiaoyu Ye <benyxy@codeaurora.org>