After resetting the hardware on a tx_timeout, call netif_wake_queue()
only if we have free tx descriptors.

Also, attempt to recover if mv643xx_eth_start_xmit() is called when
there are fewer free tx descriptors than expected.

The BUG_ON() call we are replacing was hit on a tx_timeout that
called netif_wake_queue(), indirectly via netif_device_attach(),
even though we did not have enough free tx descriptors.
Signed-off-by: Dale Farnsworth <dale@farnsworth.org>
Signed-off-by: Jeff Garzik <jeff@garzik.org>