regulator: rc5t583: Simplify rc5t583_set_voltage_time_sel implementation

For linear mappings, we can use below equation to get the voltage difference
between new_selector and old_selector:

abs(new_selector - old_selector) * rdev->desc->uV_step

Signed-off-by: Axel Lin <axel.lin@gmail.com>
Acked-by: Laxman Dewangan <ldewangan@nvidia.com>
Signed-off-by: Mark Brown <broonie@opensource.wolfsonmicro.com>