regulator: rpmh-regulator: update ARC voltage level masking logic

Change the logic applied for ARC regulator resources that define
hardware voltage level 0 as "off" in order to avoid sending an
incorrect regulator disable request when regulator_set_voltage()
is called before regulator_enable().

Change-Id: If3fe9e088d6ee875261381f00a0eaa32960e3199
Signed-off-by: David Collins <collinsd@codeaurora.org>