Fix 'usTimeOffset' was not declared error

When enable 'bios-post-code-log' feature will cause 'usTimeOffset'
was not declared error, therefore changing the scope of 'usTimeOffset'
variable.

Tested:
    - Tested build OK.
    - Check /var/log/redfish OK.
      1970-01-01T00:02:38.626333+00:00 OpenBMC.0.1.BIOSPOSTCode,2,0.0000,0x01
      1970-01-01T00:02:38.656143+00:00 OpenBMC.0.1.BIOSPOSTCode,2,0.0183,0x02
      1970-01-01T00:02:38.664210+00:00 OpenBMC.0.1.BIOSPOSTCode,2,0.0420,0x02

Signed-off-by: Alan Kuo <Alan_Kuo@quantatw.com>
Change-Id: I1d35777e47d507c2bba5b6b6afa72ca70cb23155
diff --git a/src/post_code.cpp b/src/post_code.cpp
index 3633ea2..1fcbe55 100644
--- a/src/post_code.cpp
+++ b/src/post_code.cpp
@@ -88,17 +88,17 @@
     else
     {
         // calculating tsUS so it is monotonic within the same boot
-        uint64_t usTimeOffset =
-            std::chrono::duration_cast<std::chrono::microseconds>(
-                postCodeTimeSteady - firstPostCodeTimeSteady)
-                .count();
-        tsUS = usTimeOffset + firstPostCodeUsSinceEpoch;
+        tsUS = firstPostCodeUsSinceEpoch +
+               std::chrono::duration_cast<std::chrono::microseconds>(
+                   postCodeTimeSteady - firstPostCodeTimeSteady)
+                   .count();
     }
 
     postCodes.insert(std::make_pair(tsUS, code));
     serialize(fs::path(strPostCodeListPath));
 
 #ifdef ENABLE_BIOS_POST_CODE_LOG
+    uint64_t usTimeOffset = tsUS - firstPostCodeUsSinceEpoch;
     std::ostringstream hexCode;
     hexCode << "0x" << std::setfill('0') << std::setw(2) << std::hex
             << std::get<0>(code);
@@ -114,7 +114,7 @@
     phosphor::logging::log<phosphor::logging::level::INFO>(
         "BIOS POST Code",
         phosphor::logging::entry("REDFISH_MESSAGE_ID=%s",
-                                 "OpenBMC.0.1.BIOSPOSTCode"),
+                                 "OpenBMC.0.2.BIOSPOSTCode"),
         phosphor::logging::entry(
             "REDFISH_MESSAGE_ARGS=%d,%s,%s", currentBootCycleIndex,
             timeOffsetStr.str().c_str(), hexCode.str().c_str()));