regulator: palmas: Fix calcuating selector in palmas_map_voltage_smps

The logic of calculating selector in palmas_map_voltage_smps() does not match
the logic to list voltage in palmas_list_voltage_smps().

We use below equation to calculate voltage when selector > 0:
        voltage = (0.49V + (selector * 0.01V)) * RANGE
RANGE is either x1 or x2

So we need to take into account with the multiplier set in VSEL register when
calculating selector in palmas_map_voltage_smps()

Signed-off-by: Axel Lin <axel.lin@gmail.com>
Acked-by: Graeme Gregory <gg@slimlogic.co.uk>
Signed-off-by: Mark Brown <broonie@opensource.wolfsonmicro.com>