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: I9e1c7447883c0d80b47951ecb661304b8e8e84da
diff --git a/src/inventory_mac.cpp b/src/inventory_mac.cpp
index 67e8dff..fb15e67 100644
--- a/src/inventory_mac.cpp
+++ b/src/inventory_mac.cpp
@@ -103,8 +103,7 @@
         }
     }();
 
-    ObjectTree objectTree;
-    mapperReply.read(objectTree);
+    auto objectTree = mapperReply.unpack<ObjectTree>();
 
     if (objectTree.empty())
     {
@@ -164,8 +163,8 @@
         }
     }();
 
-    std::variant<std::string> value;
-    reply.read(value);
+    auto value = reply.unpack<std::variant<std::string>>();
+
     return stdplus::fromStr<stdplus::EtherAddr>(std::get<std::string>(value));
 }