fbdev: hitfb: Fix integer-to-pointer cast

Fix the type casting from unsigned long to char __iomem *. Resolves
the following warning:

../drivers/video/fbdev/hitfb.c:411:27: warning: incorrect type in assignment (different address spaces)
../drivers/video/fbdev/hitfb.c:411:27:    expected char [noderef] __iomem *screen_base
../drivers/video/fbdev/hitfb.c:411:27:    got void *

Signed-off-by: Thomas Zimmermann <tzimmermann@suse.de>
Reviewed-by: Michael J. Ruhl <michael.j.ruhl@intel.com>
Signed-off-by: Helge Deller <deller@gmx.de>