regulator: ltc3589: Remove ltc3589_list_voltage_fixed function

When fixed_uV is set and n_voltage is 1, regulator core will return
rdev->desc->fixed_uV in regulator_get_voltage() and regulator_list_voltage().

Rename ltc3589_standby_regulator_ops to ltc3589_fixed_standby_regulator_ops,
this makes the code clear that the ops is for fixed voltage regulator.

Signed-off-by: Axel Lin <axel.lin@ingics.com>
Acked-by: Philipp Zabel <p.zabel@pengutronix.de>
Signed-off-by: Mark Brown <broonie@linaro.org>