regulator: qcom-rpmh: Implement get_optimum_mode(), not set_load()

Since we don't actually pass the load to the firmware, switch to using
get_optimum_mode() instead of open-coding it.

This is intended to have no effect other than cleanup.

Signed-off-by: Douglas Anderson <dianders@chromium.org>
Link: https://lore.kernel.org/r/20220726102024.1.Icc838fe7bf0ef54a014ab2fee8af311654f5342a@changeid


Signed-off-by: Mark Brown <broonie@kernel.org>