ASoC: cs42l51: Convert to SYSTEM_SLEEP_PM_OPS()

ASoC: cs42l51: Convert to SYSTEM_SLEEP_PM_OPS()

Use the newer SYSTEM_SLEEP_PM_OPS() macro instead of
SET_SYSTEM_SLEEP_PM_OPS().

Also, drop superfluous __maybe_unused attributes to cs42l51_suspend()
and _resume() functions.

Merely a cleanup, there should be no actual code change.

Cc: patches@opensource.cirrus.com
Reviewed-by: Charles Keepax <ckeepax@opensource.cirrus.com>
Signed-off-by: Takashi Iwai <tiwai@suse.de>
Link: https://patch.msgid.link/20250317095603.20073-11-tiwai@suse.de
Signed-off-by: Mark Brown <broonie@kernel.org>