mv643xx_eth: also check TX_IN_PROGRESS when disabling transmit path

The recommended sequence for waiting for the transmit path to clear
after disabling all of the transmit queues is to wait for the
TX_FIFO_EMPTY bit in the Port Status register to become set as well
as the TX_IN_PROGRESS bit to clear.

Signed-off-by: Lennert Buytenhek <buytenh@marvell.com>