Access through mmapped memory is not supported and it is explicitly
disabled with scontrol->readback_offset = 0; when a control is created.

Remove the dead code and the confusion around this feature.
Signed-off-by: Peter Ujfalusi <peter.ujfalusi@linux.intel.com>
Reviewed-by: Pierre-Louis Bossart <pierre-louis.bossart@linux.intel.com>
Reviewed-by: Kai Vehmanen <kai.vehmanen@intel.com>
Signed-off-by: Ranjani Sridharan <ranjani.sridharan@linux.intel.com>
Link: https://lore.kernel.org/r/20220330201926.1330402-6-ranjani.sridharan@linux.intel.comSigned-off-by: Mark Brown <broonie@kernel.org>