int mc32_close (struct net_device * dev);
The 3c527 is a bus mastering device. We must be careful how we shut it down. It may also be running shared interrupt so we have to be sure to silence it properly
We indicate that the card is closing to the rest of the driver. Otherwise, it is possible that the card may run out of receive buffers and restart the transceiver while we're trying to close it.
We abort any receive and transmits going on and then wait until any pending exec commands have completed in other code threads. In theory we can't get here while that is true, in practice I am paranoid
We turn off the interrupt enable for the board to be sure it can't intefere with other devices.