mv643xx_eth: Allocate receive queue initialized to zero

Zero pointer in rx_skb is how respective rxq_deinit() finds out out that a skb
slot is unallocated. If rxq_refill() fails (e.g. on OOM condition), subsequent
teardown would result in an attempt to kfree() invalid pointers.

Signed-off-by: Lubomir Rintel <lkundrak@v3.sk>
Cc: Lennert Buytenhek <buytenh@wantstofly.org>
Cc: netdev@vger.kernel.org
Cc: linux-kernel@vger.kernel.org
Signed-off-by: David S. Miller <davem@davemloft.net>