ACPI: fix empty macros found by -Wextra

ACPI has a ton of macros which make a bunch of empty if's when configured
in non-debug mode.

[lenb: The code it complaines about is functionally correct,
 so this patch is just to make -Wextra happier]

#define DBG()

if(...)
        DBG();
next_c_statement

which turns into
if(...) ;
next_c_statement

Signed-off-by: Dave Jones <davej@redhat.com>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Len Brown <len.brown@intel.com>

drivers/acpi/glue.c

@@ -16,7 +16,7 @@
 #if ACPI_GLUE_DEBUG
 #define DBG(x...) printk(PREFIX x)
 #else
-#define DBG(x...)
+#define DBG(x...) do { } while(0)
 #endif
 static LIST_HEAD(bus_type_list);
 static DECLARE_RWSEM(bus_type_sem);