regulator: core: simplify return value on suported_voltage

All the current clients of this API  assume that 0 corresponds
to a failure and non-zero to a pass therefore ignoring the need to
handle a negative error code.

This commit modifies the API to follow that standard since returning a
negative (EINVAL) doesn't seem to provide enough value to justify
the need to handle it.

Signed-off-by: Jorge Ramirez-Ortiz <jorge.ramirez-ortiz@linaro.org>
Signed-off-by: Mark Brown <broonie@kernel.org>