pxa168_eth: Allocate receive queue initialized to zero

Zero pointer in rx_skb or tx_skb is how respective *_deinit() functions find
out that a skb slot is unallocated. If *_init() functions unsuccessfully return
after the allocation (e.g. when subsequent dma_alloc_coherent() is not
successful), this would result in attempt to kfree() invalid pointers.

Signed-off-by: Lubomir Rintel <lkundrak@v3.sk>
Cc: Kosta Zertsekel <konszert@marvell.com>
Cc: netdev@vger.kernel.org
Cc: linux-kernel@vger.kernel.org
Signed-off-by: David S. Miller <davem@davemloft.net>