net: tile: Use helpers from linux/etherdevice.h to check/set MAC

Use is_zero_ether_addr() to check for the MAC address being all zeros instead of
open coding the check.

Also use ether_addr_copy() instead of a manual memcpy() to set the
netdev->dev_addr.

Furthermore, get rid of a redundant assignment of netdev->addr_len. This is
already set by ether_setup() which is called in tile_net_setup().

Signed-off-by: Tobias Klauser <tklauser@distanz.ch>
Acked-by: Chris Metcalf <cmetcalf@tilera.com>
Signed-off-by: David S. Miller <davem@davemloft.net>