Staging: dgnc: Replace printk with dev_err

This patch fixes the following checkpatch.pl warning:
WARNING: Prefer [subsystem eg: netdev]_err over printk(KERN_ERR, ...).

Signed-off-by: Cristina Opriceana <cristina.opriceana@gmail.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>