regulator: fixed: fix regulator_list_voltage() for regression

regulator: fixed: fix regulator_list_voltage() for regression

Commit c368e5fc2a190923b786f2de3e79430ea3566a25 "regulator: fixed:
get rid of {get|list}_voltage()" broke regulator_list_voltage() for
the fixed regulator, because an earlier commit
5a523605afa7d3b54b2e7041f8c9e6bc39872a7e "regulator: core: provide
fixed voltage in desc for single voltage rail" missed to add support
for the fixed-voltage special case to that function. This patch
fixes that regression.

Signed-off-by: Guennadi Liakhovetski <g.liakhovetski@gmx.de>
Acked-by: Laxman Dewangan <ldewangan@nvidia.com>
Signed-off-by: Mark Brown <broonie@linaro.org>