list_first_entry() always returns non-null here.  I think the intent was
to test whether there were any entries in the used list.
Signed-off-by: Dan Carpenter <error27@gmail.com>
Cc: Steven Toth <stoth@kernellabs.com>
Signed-off-by: Mauro Carvalho Chehab <mchehab@redhat.com>