EMAC driver: fix bug - invalidate data cache of new_skb->data range when cache is WB

It prevents overwritting new data from DMA.

Signed-off-by: Alexey Demin <bf53x@ya.ru>
Signed-off-by: Bryan Wu <bryan.wu@analog.com>
Signed-off-by: Jeff Garzik <jeff@garzik.org>
Signed-off-by: David S. Miller <davem@davemloft.net>