ASoC: SOF: ipc4-topology: Correct data structures for the GAIN module

Move the base_cfg to struct sof_ipc4_gain_data. This struct
describes the message payload passed to the firmware via the mailbox.

It is not wise to be 'clever' and try to use the first part of a struct
as IPC message without marking the message section as packed and aligned.

Signed-off-by: Peter Ujfalusi <peter.ujfalusi@linux.intel.com>
Reviewed-by: Pierre-Louis Bossart <pierre-louis.bossart@linux.intel.com>
Reviewed-by: Bard Liao <yung-chuan.liao@linux.intel.com>
Reviewed-by: Ranjani Sridharan <ranjani.sridharan@linux.intel.com>
Link: https://lore.kernel.org/r/20231129131411.27516-3-peter.ujfalusi@linux.intel.com
Signed-off-by: Mark Brown <broonie@kernel.org>