ALSA: cs35l41: Use mbox command to enable speaker output for external boost

To enable the speaker output in external boost mode, 2 registers must
be set, one after another. The longer the time between the writes of
the two registers, the more likely, and more loudly a pop may occur.
To minimize this, an mbox command can be used to allow the firmware
to perform this action, minimizing any delay between write, thus
minimizing any pop or click as a result. The old method will remain
when running without firmware.

Acked-by: Mark Brown <broonie@kernel.org>
Signed-off-by: Stefan Binding <sbinding@opensource.cirrus.com>
Link: https://lore.kernel.org/r/20230721151816.2080453-2-sbinding@opensource.cirrus.com
Signed-off-by: Takashi Iwai <tiwai@suse.de>