syslog_default_write: Fix the return value

Found by clang-check:

syslog/syslog_write.c:96:7: warning: Value stored to 'nwritten' is never read
      nwritten = g_syslog_channel->sc_write(buffer, buflen);
      ^          ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
1 warning generated.

drivers/syslog/syslog_write.c

@@ -105,7 +105,7 @@ static ssize_t syslog_default_write(FAR const char *buffer, size_t buflen)
         }
     }
 
-  return buflen;
+  return nwritten;
 }
 
 /****************************************************************************