regulator: Set delay to 0 if set_voltage_time_sel callback returns error

rdev->desc->ops->set_voltage_time_sel may return negative error code.
Set delay to 0 and also show warning if set_voltage_time_sel returns error.

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