powerpc/sysdev: fix incorrect use to determine if list is empty

'gtm' will *always* be set by list_for_each_entry().
It is incorrect to assume that the iterator value will be NULL if the
list is empty.

Instead of checking the pointer it should be checked if
the list is empty.

Fixes: 83ff9dcf ("powerpc/sysdev: implement FSL GTM support")
Signed-off-by: Jakob Koschel <jakobkoschel@gmail.com>
Signed-off-by: Michael Ellerman <mpe@ellerman.id.au>
Link: https://lore.kernel.org/r/20220228142434.576226-1-jakobkoschel@gmail.com

arch/powerpc/sysdev/fsl_gtm.c

@@ -86,7 +86,7 @@ static LIST_HEAD(gtms);
  */
 struct gtm_timer *gtm_get_timer16(void)
 {
-	struct gtm *gtm = NULL;
+	struct gtm *gtm;
 	int i;
 
 	list_for_each_entry(gtm, &gtms, list_node) {
@@ -103,7 +103,7 @@ struct gtm_timer *gtm_get_timer16(void)
 		spin_unlock_irq(&gtm->lock);
 	}
 
-	if (gtm)
+	if (!list_empty(&gtms))
 		return ERR_PTR(-EBUSY);
 	return ERR_PTR(-ENODEV);
 }