regulator: qcom_smd: Remove list_voltage callback for rpm_smps_ldo_ops_fixed

Use regulator_list_voltage_linear_range in rpm_smps_ldo_ops_fixed is
wrong because it is used for fixed regulator without any linear range.
The rpm_smps_ldo_ops_fixed is used for pm8941_lnldo which has fixed_uV
set and n_voltages = 1. In this case, regulator_list_voltage() can return
rdev->desc->fixed_uV without .list_voltage implementation.

Fixes: 3bfbb4d1 ("regulator: qcom_smd: add list_voltage callback")
Signed-off-by: Axel Lin <axel.lin@ingics.com>
Signed-off-by: Mark Brown <broonie@kernel.org>