[style] harmonize boolean checks (#4048)

This commit simplifies boolean checks containing `== false` to use the
`!` operator instead, and removes redundant `== true` checks. This
commit aims to harmonize the boolean checks across core modules.

src/cli/cli.cpp

@@ -1023,7 +1023,7 @@ void Interpreter::ProcessEidCache(int argc, char *argv[])
     {
         SuccessOrExit(otThreadGetEidCacheEntry(mInstance, i, &entry));
 
-        if (entry.mValid == false)
+        if (!entry.mValid)
         {
             continue;
         }