[PATCH] mv643xx_eth: Rename mp->tx_ring_skbs to mp->tx_desc_count

tx_ring_skbs is actually a count of tx descriptors currently in use.
Since there may be multiple descriptors per skb, it is not the
same as the number of skbs in the ring.

Also change rx_ring_skbs to rx_desc_count to be consistent.

Signed-off-by: Dale Farnsworth <dale@farnsworth.org>
Signed-off-by: Jeff Garzik <jgarzik@pobox.com>