mv643xx_eth: mp->tx_desc_count needs spinlock protection

mv643xx_eth_start_xmit() should check mp->tx_desc_count only
inside the mp->lock spinlock.

Signed-off-by: Lennert Buytenhek <buytenh@marvell.com>
Reviewed-by: Tzachi Perelstein <tzachi@marvell.com>
Signed-off-by: Dale Farnsworth <dale@farnsworth.org>