regulator: palmas: Simplify code by not indexing regulator_desc unnecessarily

Palmas regulator needs to full up the regulator_desc based on PMIC and
type of regulator. However, we dont need to do desc[id] every time. we
can simplify by using a pointer to desc[id] and filling up the
parameters.
Signed-off-by: Nishanth Menon <nm@ti.com>
Tested-by: Stephen Warren <swarren@nvidia.com>
Tested-by: Keerthy <j-keerthy@ti.com>
Signed-off-by: Mark Brown <broonie@linaro.org>