usb/kaweth: print MAC via printk format specifier

Print MAC/dev_addr via printk extended format specifier %pM
instead of custom code.

Signed-off-by: Danny Kukawka <danny.kukawka@bisect.de>
Acked-by: Oliver Neukum <oliver@neukum.name>
Signed-off-by: David S. Miller <davem@davemloft.net>