ncsi: log interface types using to_string

Add a to_string() method for Interfaces, to handle printing via lg2.

Since we're printing the interface object, don't specify _INDEX on the
metadata, this may not always be an index.

Also, interface indices are typically referred to as decimal. Use
decimal instead of a lg2::hex conversion.

Change-Id: I90b5fbac52adde3bbc3f2699087373ed96361083
Signed-off-by: Jeremy Kerr <jk@codeconstruct.com.au>
diff --git a/src/ncsi_util.hpp b/src/ncsi_util.hpp
index da45a38..a04e249 100644
--- a/src/ncsi_util.hpp
+++ b/src/ncsi_util.hpp
@@ -1,6 +1,7 @@
 #pragma once
 
 #include <span>
+#include <string>
 
 namespace phosphor
 {
@@ -17,6 +18,8 @@
     int ifindex;
 };
 
+std::string to_string(Interface& interface);
+
 /* @brief  This function will ask underlying NCSI driver
  *         to send an OEM command (command type 0x50) with
  *         the specified payload as the OEM data.