[POWERPC] cell: fix bugs found by sparse

- Some long constants should be marked 'ul'.
- When using desc->handler_data to pass an __iomem
  register area, we need to add casts to and from
  __iomem.

Signed-off-by: Arnd Bergmann <arnd.bergmann@de.ibm.com>
Signed-off-by: Paul Mackerras <paulus@samba.org>