regulator: max8997: limit the number of dvs registers programmed in non-dvs mode

In case the gpio based volatage selection mode is not used for either of
buck 1/2/5, then only the BUCKxDVS1 register need to be programmed. So
determine whether dvs mode is used and limit the loop count appropriately.

Signed-off-by: Thomas Abraham <thomas.abraham@linaro.org>
Reviewed-by: Tomasz Figa <t.figa@samsung.com>
Signed-off-by: Mark Brown <broonie@opensource.wolfsonmicro.com>