mv643xx_eth: work around TX hang hardware issue

Under some conditions, the TXQ ('TX queue being served') bit can clear
before all packets queued for that TX queue have been transmitted.
This patch enables TXend interrupts, and uses those to re-kick TX
queues that claim to be idle but still have queued descriptors from
the interrupt handler.

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