ethernet/arc/arc_emac: add missing platform_set_drvdata() in arc_emac_probe()

Add missing platform_set_drvdata() in arc_emac_probe(), otherwise
calling platform_get_drvdata() in arc_emac_remove() may returns NULL.

Signed-off-by: Wei Yongjun <yongjun_wei@trendmicro.com.cn>
Signed-off-by: David S. Miller <davem@davemloft.net>