regulator: fixed: fix regulator_list_voltage() for regression

Commit c368e5fc "regulator: fixed:
get rid of {get|list}_voltage()" broke regulator_list_voltage() for
the fixed regulator, because an earlier commit
5a523605 "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>