mfd: cros_ec: Send correct suspend/resume event to EC

pm_suspend_via_firmware() will return false for platforms with ACPI
disabled and ACPI is a prerequisite for S0ix support.

With this patch, sleep state event sent to EC is forced to S3 if ACPI is
disabled.

Signed-off-by: Shawn Nematbakhsh <shawnn@chromium.org>
Signed-off-by: Thierry Escande <thierry.escande@collabora.com>
Signed-off-by: Lee Jones <lee.jones@linaro.org>