ppp: Clean up kernel log messages.

Use netdev_*() and pr_*().

To preserve existing semantics in cases where KERN_DEBUG is indeed
appropriate, use netdev_printk(KERN_DEBUG, ...)

Convert PPPIOCDETACH to pr_warn() because an unexpected file count is
a serious bug and should be logged with KERN_WARN.

Signed-off-by: David S. Miller <davem@davemloft.net>
Signed-off-by: Paul Mackerras <paulus@samba.org>