audio: hal: Avoid backend configuration for afe-proxy device

During A2DP src usecase with non-default sample rate,
DEFAULT_BACKEND_PORT is getting configured with new
sample rate. After the A2DP usecase when we start new
session of normal playback with default sample rate,
no audio is heard in speakers in LE targets.
To avoid issue, returning from platform_check_codec_
backend_cfg api without configuring backend.

Change-Id: I35670b3c0bf197ac80cc92c22bb73467e277a5cd