asm-generic/io.h: check CONFIG_VIRT_TO_BUS

Make asm-generic/io.h check CONFIG_VIRT_TO_BUS before defining
virt_to_bus() and bus_to_virt(), otherwise it's easy to accidentally
have a silently failing incorrect direct mapped definition rather then
no definition at all.
Signed-off-by: James Hogan <james.hogan@imgtec.com>
Acked-by: Arnd Bergmann <arnd@arndb.de>

include/asm-generic/io.h

@@ -332,6 +332,7 @@ extern void ioport_unmap(void __iomem *p);
 #define xlate_dev_kmem_ptr(p)	p
 #define xlate_dev_mem_ptr(p)	__va(p)
 
+#ifdef CONFIG_VIRT_TO_BUS
 #ifndef virt_to_bus
 static inline unsigned long virt_to_bus(volatile void *address)
 {
@@ -343,6 +344,7 @@ static inline void *bus_to_virt(unsigned long address)
 	return (void *) address;
 }
 #endif
+#endif
 
 #ifndef memset_io
 #define memset_io(a, b, c)	memset(__io_virt(a), (b), (c))