ASoC: generic-dmaengine-pcm: Use managed buffer allocation

Clean up the driver with the new managed buffer allocation API.
The hw_free callback became superfluous and got dropped.

Cc: Lars-Peter Clausen <lars@metafoo.de>
Signed-off-by: Takashi Iwai <tiwai@suse.de>
Link: https://lore.kernel.org/r/20191210142614.19405-11-tiwai@suse.de


Signed-off-by: Mark Brown <broonie@kernel.org>