include/linux/netdevice.h: Add netif_printk helpers

Add macros to test a private structure for msg_enable bits
and the netif_msg_##bit to test and call netdev_printk if set

Simplifies logic in callers and adds message logging consistency

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