drivers: net: cisco_hdlc: use __func__ in debug message

Use __func__ to print the function name instead of hard coded string.
BTW, replace printk(KERN_DEBUG, ...) with netdev_dbg.

Signed-off-by: Chen Zhou <chenzhou10@huawei.com>
Signed-off-by: David S. Miller <davem@davemloft.net>