use sdbusplus unpack syntax

Rather than defining a variable and then reading it from a message,
sdbusplus also supports directly unpack-ing from the message.  Use
this syntax instead as it is more efficient and succinct.

Signed-off-by: Patrick Williams <patrick@stwcx.xyz>
Change-Id: I59b1cef85e4e0e5dd14b33a4e1bc15aaef2a8aac
diff --git a/util.cpp b/util.cpp
index e106d26..f2205d2 100644
--- a/util.cpp
+++ b/util.cpp
@@ -104,8 +104,8 @@
         properties.append("xyz.openbmc_project.State.Chassis");
         properties.append("CurrentPowerState");
         auto result = bus.call(properties);
-        std::variant<std::string> val;
-        result.read(val);
+        auto val = result.unpack<std::variant<std::string>>();
+
         if (auto pVal = std::get_if<std::string>(&val))
         {
             powerState = *pVal;