staging: dgnc: use dev_err() instead of printk()

Use dev_err() insted of printk() in order to provice userspace with
more useful information and use the common kernel coding style.

Signed-off-by: Martin Kepplinger <martink@posteo.de>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>