tun: Convert logging messages to pr_<level> and tun_debug

Use the current logging forms with pr_fmt.
Convert DBG macro to tun_debug, use netdev_printk as well.
Add printf verification when TUN_DEBUG not defined.
Miscellaneous comment typo fix.

Signed-off-by: Joe Perches <joe@perches.com>
Signed-off-by: David S. Miller <davem@davemloft.net>