regulator: core: Return error in get optional stub

Drivers that call regulator_get_optional are tolerant to the absence of
that regulator.  By modifying the value returned from the stub function
to match that seen when a regulator isn't present, callers can wrap the
regulator logic with an IS_ERR based conditional even if they happen to
call regulator_is_supported_voltage.  This improves efficiency as well
as eliminates the possibility for a very subtle bug.

Signed-off-by: Tim Kryger <tim.kryger@linaro.org>
Reviewed-by: Alex Elder <elder@linaro.org>
Signed-off-by: Mark Brown <broonie@linaro.org>