greybus: audio-codec: Staticize few routines to fix build warnings

This fixes below warnings ..

greybus/audio_codec.c:20:32: warning: symbol 'find_data' was not declared. Should it be static?
greybus/audio_codec.c:955:6: warning: symbol 'gbaudio_codec_cleanup' was not declared. Should it be static?

Signed-off-by: Viresh Kumar <viresh.kumar@linaro.org>
Reviewed-by: Vaibhav Agarwal <vaibhav.agarwal@linaro.org>
Signed-off-by: Greg Kroah-Hartman <gregkh@google.com>