regulator: max8973: Set VSEL regmap ops if DVS GPIO is not set

Use regmap helpers for get_voltage_sel and set_voltage_sel ops
if the DVS GPIO is not set.

The DVS GPIO allows on the fly selection of the VSEL register
from two choices. However, if it is not set, the VSEL register
will stay fixed and we can use the regmap ops. This allows use
of the *hardware_vsel* regulator APIs.

Signed-off-by: Mikko Perttunen <mperttunen@nvidia.com>
Signed-off-by: Mark Brown <broonie@kernel.org>