dsp: firmware_update: Initialize msgbuf after argument tests

The msgbuf APIs are being reworked to improve soundness and error
reporting. Prior to introducing some new requirements on its users,
ensure its init/destroy sequences have minimal code spans.

This effort surfaced a problem with a test configuration for
encode_get_downstream_firmware_parameters_req() which was passing
an invalid transfer operation flag. Previously this was masked by a
buffer-length validation failure.

Change-Id: I7259ac86d696da425ac9d919e6864dfb238d8996
Signed-off-by: Andrew Jeffery <andrew@codeconstruct.com.au>
diff --git a/tests/dsp/firmware_update.cpp b/tests/dsp/firmware_update.cpp
index 78f81db..7c406aa 100644
--- a/tests/dsp/firmware_update.cpp
+++ b/tests/dsp/firmware_update.cpp
@@ -2284,7 +2284,7 @@
     constexpr uint8_t instanceId = 1;
     // Setup invalid transfer operation flag
     constexpr pldm_get_downstream_firmware_parameters_req params_req{
-        0x0, PLDM_ACKNOWLEDGEMENT_ONLY};
+        0x0, PLDM_GET_FIRSTPART};
     constexpr size_t payload_length =
         PLDM_GET_DOWNSTREAM_FIRMWARE_PARAMETERS_REQ_BYTES -
         1 /* inject erro length*/;