staging: vt6655: change type of PortOffset to void __iomem *

PortOffset was an unsigned long, but used as an pointer to io
memory. Sometimes it was not properly cast before use, which caused
many warning by sparse.

By updating its type to void __iomem *, and reflecting the changes
where it is needed, this removes most of those warnings.

Signed-off-by: Guillaume Clement <gclement@baobob.org>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>