regulator: Fix suspend to idle

When suspending to idle with the new suspend mode configuration support
we go through the suspend callbacks with a state of PM_SUSPEND_TO_IDLE
which we don't have regulator constraints for, causing an error.  Avoid
this and similar errors by treating missing constraints as a noop.
Reported-by: NGeert Uytterhoeven <geert+renesas@glider.be>
Tested-by: NGeert Uytterhoeven <geert+renesas@glider.be>
Signed-off-by: NMark Brown <broonie@kernel.org>

drivers/regulator/core.c

@@ -758,7 +758,7 @@ static int suspend_set_state(struct regulator_dev *rdev,
 
 	rstate = regulator_get_suspend_state(rdev, state);
 	if (rstate == NULL)
-		return -EINVAL;
+		return 0;
 
 	/* If we have no suspend mode configration don't set anything;
 	 * only warn if the driver implements set_suspend_voltage or