ASoC: compress: Derive substream from stream based on direction

Currently compress driver hardcodes direction as playback to get
substream from the stream. This results in getting the incorrect
substream for compressed capture usecase.
To fix this, remove the hardcoding and derive substream based on
the stream direction.

Signed-off-by: Satish Babu Patakokila <sbpata@codeaurora.org>
Signed-off-by: Banajit Goswami <bgoswami@codeaurora.org>
Acked-By: Vinod Koul <vinod.koul@intel.com>
Signed-off-by: Mark Brown <broonie@kernel.org>
Cc: stable@vger.kernel.org