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

Cc: Cezary Rojewski <cezary.rojewski@intel.com>
Cc: Pierre-Louis Bossart <pierre-louis.bossart@linux.intel.com>
Cc: Liam Girdwood <liam.r.girdwood@linux.intel.com>
Cc: Jie Yang <yang.jie@linux.intel.com>
Signed-off-by: Takashi Iwai <tiwai@suse.de>
Link: https://lore.kernel.org/r/20191210142614.19405-23-tiwai@suse.deSigned-off-by: Mark Brown <broonie@kernel.org>