dynamic_debug: make netif_dbg() call __netdev_printk()

Previously, netif_dbg() was using dynamic_dev_dbg() to perform
the underlying printk. Fix it to use __netdev_printk(), instead.

Cc: David S. Miller <davem@davemloft.net>
Signed-off-by: Jason Baron <jbaron@redhat.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@suse.de>