regulator: max8907: Fix n_voltages for MAX8907B SD1

For linear voltage mapping, the n_voltages is (max - min) / step + 1

Signed-off-by: Axel Lin <axel.lin@gmail.com>
Acked-by: Stephen Warren <swarren@wwwdotorg.org>
Signed-off-by: Mark Brown <broonie@opensource.wolfsonmicro.com>