os_freebsd.cpp: tiny change that fixes one logical error and couple of style deviations

git-svn-id: https://smartmontools.svn.sourceforge.net/svnroot/smartmontools/trunk@2953 4ea69e1a-61f1-4043-bf83-b5c94c648137

smartmontools/os_freebsd.cpp

@@ -227,7 +227,7 @@ bool freebsd_smart_device::close()
 {
   int failed = 0;
   // close device, if open
-  if (get_fd())
+  if (is_open())
     failed=::close(get_fd());
 
   set_fd(-1);