mv643xx_eth: be more agressive about RX refill

During OOM, instead of stopping RX refill when the rx desc ring is
not empty, keep trying to refill the ring as long as it is not full
instead.

Signed-off-by: Lennert Buytenhek <buytenh@marvell.com>
Acked-by: Dale Farnsworth <dale@farnsworth.org>