Only pre-allocate 256 bytes of cardbio IO range

It may seem small, but most cards need much less, if any, and this not
only makes the code adhere to the comment, it seems to fix a boot-time
lockup on a ThinkPad 380XD laptop reported by Tero Roponen <teanropo@cc.jyu.fi>

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