regulator: core: Change dummy supplies error message to a warning

Change "dummy supplies not allowed" error message to warning instead, as this
is a just warning message with no change to the behavior.

[Added a CC to stable since some other bug fixes cause this to come up
more frequently on PCs which is how it was noticed -- broonie]

Signed-off-by: Shuah Khan <shuah.kh@samsung.com>
Signed-off-by: Mark Brown <broonie@linaro.org>
Cc: stable@vger.kernel.org