regulator: Fix display of null constraints for regulators

If the regulator constraints are empty and there is no voltage
reported then nothing will be added to the text displayed for the
constraints, leading to random stack data being printed. This is
unlikely to happen for practical regulators since most will at
least report a voltage but should still be fixed.

Signed-off-by: Mark Brown <broonie@opensource.wolfsonmicro.com>
Cc: stable@kernel.org
Signed-off-by: Liam Girdwood <lrg@slimlogic.co.uk>