regulator: tps65910: correct init value of n_voltages

Recent change in the core driver to get the maximum voltage
is based on the (n_voltages -1) steps of voltage.
For the tps65910, the (n_voltages -1)th step voltage is
calculated based on the callback function list_voltage.
This function direct maps the datasheet and adjust the
first few steps for initial voltage as per datasheet,
and hence initialize the n_voltages based on datasheet.

Signed-off-by: Laxman Dewangan <ldewangan@nvidia.com>
Signed-off-by: Mark Brown <broonie@opensource.wolfsonmicro.com>