zorro: ZTWO_VADDR() should return "void __iomem *"

ZTWO_VADDR() converts from physical to virtual I/O addresses, so it should
return "void __iomem *" instead of "unsigned long".

This allows to drop several casts, but requires adding a few casts to
accomodate legacy driver frameworks that store "unsigned long" I/O
addresses.

Signed-off-by: Geert Uytterhoeven <geert@linux-m68k.org>