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: I23902026dee367a8cbf0c2725725c4ed3ca4d2bb
diff --git a/transport/serialbridge/serialcmd.cpp b/transport/serialbridge/serialcmd.cpp
index c6426fb..94535e5 100644
--- a/transport/serialbridge/serialcmd.cpp
+++ b/transport/serialbridge/serialcmd.cpp
@@ -163,9 +163,8 @@
             throw sdbusplus::exception::SdBusError(&error, "ipmid response");
         }
 
-        std::tuple<uint8_t, uint8_t, uint8_t, uint8_t, std::vector<uint8_t>>
-            ret;
-        m.read(ret);
+        auto ret = m.unpack<std::tuple<uint8_t, uint8_t, uint8_t, uint8_t,
+                                       std::vector<uint8_t>>>();
 
         const auto& [netFn, lun, cmd, cc, data] = ret;