korina: fix usage of driver_data

Using platform_set_drvdata() here makes no sense, since the driver_data
field has already been filled with valuable data (i.e. the MAC address).
Also having driver_data point to the net_device is rather pointless
since struct korina_device contains an apropriate field for it.

Signed-off-by: Phil Sutter <n0-1@freewrt.org>
Signed-off-by: David S. Miller <davem@davemloft.net>