staging: comedi: dt3000: io_addr is a void __iomem *

The io_addr variable in struct dt3k_private is an ioremap'ed pci resource and should be void __iomem * not a void *.

This quiets a lot of sparse warings for the writew and readw calls like:

warning: incorrect type in argument 1 (different address spaces) expected void const volatile [noderef] <asn:2>*addr got void *<noident>

warning: incorrect type in argument 2 (different address spaces) expected void volatile [noderef] <asn:2>*addr
   got void *

---

index 0d27326..71c3479 100644
@@ -254,7 +254,7 @@ struct dt3k_private {
 
 	struct pci_dev *pci_dev;
 	resource_size_t phys_addr;
-	void *io_addr;
+	void __iomem *io_addr;
 	unsigned int lock;
 	unsigned int ao_readback[2];
 	unsigned int ai_front;