psu-ng: Remove power rating prop if PS not present

When the power is turned on, the input power rating properties were
only being created or updated, and if the PS wasn't present nothing was
being done.  So if a PS was previously present but was removed, its
property would stay on D-Bus.

Fix that by deleting the interface if the PS isn't present at the time
of the check.

As PLDM is listening for this and it doesn't handle InterfacesRemoved
signals, also set the value to zero before removing it so that they will
at least see a value of zero for the missing PS.

At the time of this writing, the only consumer of this property was
hostboot (the BIOS for IBM systems) via PLDM, and they were also
checking if the PS was present before using it so technically they
weren't broken, but it's good to fix it anyway because it's misleading.

Change-Id: I708014bdb12288d95e77028186b38a812f87e916
Signed-off-by: Matt Spinler <spinler@us.ibm.com>
diff --git a/phosphor-power-supply/power_supply.cpp b/phosphor-power-supply/power_supply.cpp
index b362a57..3fd72e8 100644
--- a/phosphor-power-supply/power_supply.cpp
+++ b/phosphor-power-supply/power_supply.cpp
@@ -1186,6 +1186,11 @@
 {
     if (!present)
     {
+        if (inputVoltageRatingIface)
+        {
+            inputVoltageRatingIface->value(0);
+            inputVoltageRatingIface.reset();
+        }
         return;
     }