ASoC: cs35l41: Move cs35l41_set_cspl_mbox_cmd to shared code

This function is used to control the DSP Firmware for cs35l41,
and will be needed by the cs35l41 hda driver, when firmware
support is added.

Signed-off-by: Stefan Binding <sbinding@opensource.cirrus.com>
Signed-off-by: Vitaly Rodionov <vitalyr@opensource.cirrus.com>
Acked-by: Charles Keepax <ckeepax@opensource.cirrus.com>
Link: https://lore.kernel.org/r/20220509214703.4482-7-vitalyr@opensource.cirrus.com
Signed-off-by: Takashi Iwai <tiwai@suse.de>