gpio-ml-ioh: cleanup __iomem annotation usage

The __iomem annotation is to be used together with pointers used
in iowrite32() but not for pointers returned by kzalloc().
For more details see [1] and [2].

This patch will remove the following sparse warning (i.e. when
copiling with "make C=1"):
 * warning: incorrect type in assignment (different address spaces)

References:
[1] A new I/O memory access mechanism (Sep 15, 2004)
    http://lwn.net/Articles/102232/

[2] Being more anal about iospace accesses (Sep 15, 2004)
    http://lwn.net/Articles/102240/

Signed-off-by: Márton Németh <nm127@freemail.hu>
Signed-off-by: Grant Likely <grant.likely@secretlab.ca>