physical: Fix bugprone-narrowing-conversions

```
/home/andrew/src/openbmc/phosphor-led-sysfs/build/../physical.cpp:138:14: error: narrowing conversion from 'int' to signed type '__gnu_cxx::__alloc_traits<std::allocator<char>, char>::value_type' (aka 'char') is implementation-defined [bugprone-narrowing-conversions,cppcoreguidelines-narrowing-conversions,-warnings-as-errors]
    tmp[0] = toupper(tmp[0]);
             ^
```

Change-Id: I58c9a47008272b856427419557dc3e592936b5eb
Signed-off-by: Andrew Jeffery <andrew@aj.id.au>
diff --git a/physical.cpp b/physical.cpp
index f62bca5..d060135 100644
--- a/physical.cpp
+++ b/physical.cpp
@@ -135,7 +135,7 @@
     }
 
     std::string tmp = color;
-    tmp[0] = toupper(tmp[0]);
+    tmp[0] = static_cast<char>(toupper(tmp[0]));
     try
     {
         auto palette = convertPaletteFromString(prefix + tmp);