regulator: max77686: Fix the delay time for set_voltage_time_sel

rdev->desc->uV_step * abs(new_selector - old_selector) returns uV.
The unit of ramp_rate is mV/us.
Thus 1000 should be multiplied.

Signed-off-by: Axel Lin <axel.lin@gmail.com>
Reviewed-by: Yadwinder Singh Brar <yadi.brar@samsung.com>
Signed-off-by: Mark Brown <broonie@opensource.wolfsonmicro.com>