staging: dgnc_sysfs: Replace printk(KERN_ERR ) with pr_err()

Fix the following checkpatch warning:
WARNING: Prefer [subsystem eg: netdev]_err([subsystem]dev, ...
then dev_err(dev, ... then pr_err(...  to printk(KERN_ERR ...

Signed-off-by: Ioana Ciornei <ciorneiioana@gmail.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>