cardbus: limit IO windows to 256 bytes

That's what we've always historically done, and bigger windows seem to
confuse some cardbus bridges. Or something.

Alan reports that this makes the ThinkPad 600x series work properly
again: the 4kB IO window for some reason made IDE DMA not work, which
makes IDE painfully slow even if it works after DMA timeouts.

Signed-off-by: Linus Torvalds <torvalds@osdl.org>