physical: Fix readability-implicit-bool-conversion

```
/home/andrew/src/openbmc/phosphor-led-sysfs/build/../physical.cpp:46:13: error: implicit conversion 'unsigned long' -> bool [readability-implicit-bool-conversion,-warnings-as-errors]
        if (brightness && assert)
            ^
            (          != 0u)
/home/andrew/src/openbmc/phosphor-led-sysfs/build/../physical.cpp:46:27: error: implicit conversion 'unsigned long' -> bool [readability-implicit-bool-conversion,-warnings-as-errors]
        if (brightness && assert)
                          ^
                          (      != 0u)
```

Change-Id: I556d7d24cd1bd6372285d58c78fd1b17339b2ebe
Signed-off-by: Andrew Jeffery <andrew@aj.id.au>
diff --git a/physical.cpp b/physical.cpp
index f4ef8a1..f488a41 100644
--- a/physical.cpp
+++ b/physical.cpp
@@ -43,7 +43,7 @@
     {
         // Cache current LED state
         auto brightness = led.getBrightness();
-        if (brightness && assert)
+        if (brightness != 0U && assert != 0U)
         {
             sdbusplus::xyz::openbmc_project::Led::server::Physical::state(
                 Action::On);