drivers: net: Don't print unpopulated net_device name

For ethernet devices, net_device.name will be eth%d before
register_netdev() is called. Don't print the net_device name until
the format string is replaced.

Signed-off-by: Harvey Hunt <harvey.hunt@imgtec.com>
Cc: Marcel Ziswiler <marcel@ziswiler.com>
Cc: Robert Jarzmik <robert.jarzmik@free.fr>
Cc: Barry Song <Baohua.Song@csr.com>
Cc: netdev@vger.kernel.org
Cc: linux-kernel@vger.kernel.org
Signed-off-by: David S. Miller <davem@davemloft.net>