regulator: rpm-smd-regulator: don't wait for ACK when lowering power state

Modify the rpm-smd-regulator driver so that it does not wait for
an ACK or a NACK from the RPM after sending a request for a
strictly lower power state.  Some keys represent attributes with
well defined higher/lower semantics.  These include 'swen', 'uv',
'ma', 'corn', and 'vlvl'.  There is no need to wait for an ACK if
the value of one of the keys from this set is being configured to
a lower value.  Some keys such as 'pcen', 'pcmd', and 'resn' do
not have higher/lower semantics so an ACK is required any time
their value changes.

Change-Id: I3a04e4ee81cf8d193eb48e37e80511fed62b0a6c
Signed-off-by: David Collins <collinsd@codeaurora.org>