[media] mem2mem_testdev: Use pr_err instead of printk

printk(KERN_ERR...) is replaced with pr_err to silence checkpatch
warning.
WARNING: Prefer netdev_err(netdev, ... then dev_err(dev, ...
then pr_err(...  to printk(KERN_ERR ...
Signed-off-by: Sachin Kamat <sachin.kamat@linaro.org>
Signed-off-by: Mauro Carvalho Chehab <mchehab@redhat.com>

drivers/media/platform/mem2mem_testdev.c

@@ -397,8 +397,7 @@ static void device_isr(unsigned long priv)
 	curr_ctx = v4l2_m2m_get_curr_priv(m2mtest_dev->m2m_dev);
 
 	if (NULL == curr_ctx) {
-		printk(KERN_ERR
-			"Instance released before the end of transaction\n");
+		pr_err("Instance released before the end of transaction\n");
 		return;
 	}