regulator: prevent registration of matching regulator consumer supplies

Acked-by: Mark Brown <broonie@opensource.wolfsonmicro.com>
Pointer comparison is not sufficient for non-NULL device name matching,
so use strcmp(). Otherwise the semantics remain the same.

Signed-off-by: Jani Nikula <ext-jani.1.nikula@nokia.com>
Signed-off-by: Liam Girdwood <lrg@slimlogic.co.uk>