fixup: CI is x86-64, build is 32, cast down for debug

The fprintfs were promoted to %lu to make CI work, which then broke the
ARM build.  Therefore need to cast to make it happy in both
environments.

Change-Id: Ibba14d8aa19aca44c9ce9b65b66dd79659ce126e
Signed-off-by: Patrick Venture <venture@google.com>
diff --git a/eth.cpp b/eth.cpp
index 32dbe89..ba46f73 100644
--- a/eth.cpp
+++ b/eth.cpp
@@ -70,7 +70,8 @@
 {
     if ((*dataLen) < sizeof(struct EthDeviceRequest))
     {
-        fprintf(stderr, "Invalid command length: %lu\n", (*dataLen));
+        fprintf(stderr, "Invalid command length: %u\n",
+                static_cast<uint32_t>(*dataLen));
         return IPMI_CC_INVALID;
     }