mv643xx_eth: prevent interrupt storm on ifconfig down

Contrary to what the docs say, the 'extended interrupt cause' bit in
the interrupt cause register (bit 1) appears to not be maskable on at
least some of the mv643xx_eth platforms, making writing zeroes to the
interrupt mask register but not the extended interrupt mask register
insufficient to stop interrupts from occuring.  Therefore, also write
zeroes to the extended interrupt mask register when shutting down the
port.

This fixes the interrupt storm seen on the Pegasos board when shutting
down the interface.

Signed-off-by: Lennert Buytenhek <buytenh@marvell.com>
Signed-off-by: David S. Miller <davem@davemloft.net>