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/network_manager.cpp b/src/network_manager.cpp
index 75c759b..5991611 100644
--- a/src/network_manager.cpp
+++ b/src/network_manager.cpp
@@ -154,8 +154,8 @@
                                       "org.freedesktop.DBus.Properties", "Get");
         req.append("org.freedesktop.network1.Link", "AdministrativeState");
         auto rsp = req.call();
-        std::variant<std::string> val;
-        rsp.read(val);
+        auto val = rsp.unpack<std::variant<std::string>>();
+
         handleAdminState(std::get<std::string>(val), ifidx);
     }