mv643xx_eth: use the SMI done interrupt to wait for SMI access completion

If the platform code has passed us the IRQ number of the mv643xx_eth
top-level error interrupt, use the error interrupt to wait for SMI
access completion instead of polling the SMI busy bit, since SMI bus
accesses can take up to tens of milliseconds.

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