[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>