pxa168_eth: silence gcc warnings

Casting "pep->tx_desc_dma" to to a struct tx_desc pointer makes gcc
complain:

drivers/net/pxa168_eth.c:657: warning:
	cast to pointer from integer of different size

Signed-off-by: Dan Carpenter <error27@gmail.com>
Signed-off-by: David S. Miller <davem@davemloft.net>