staging:dgnc: Replace printk by pr_warn

This patch replaces printk with pr_warn as the printk calls are used to
display warnings to remove the checkpatch.pl warnings:
WARNING: printk() should include KERN_ facility level
and then WARNING: Prefer netdev_warn(netdev, ... then dev_warn(dev,
... then pr_warn(...  to printk(KERN_WARNING ... on adding KERN_WARNING
message designation.

Signed-off-by: Himangi Saraogi <himangi774@gmail.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>