regulator: make the dummy regulator's print_constraint more helpful

This prevents the output of just

	dummy:

in the boot log. Now it says:

	regulator-dummy: no parameters

which at least doesn't make it look like an accidental printk and also doesn't
only use "dummy" which could mean anything.

Signed-off-by: Uwe Kleine-König <u.kleine-koenig@pengutronix.de>
Signed-off-by: Mark Brown <broonie@opensource.wolfsonmicro.com>