staging: don't use module_init in non-modular ion_dummy_driver.c

The ION_DUMMY option is bool, and hence this code is either
present or absent. It will never be modular, so using
module_init as an alias for __initcall is rather misleading.

Fix this up now, so that we can relocate module_init from
init.h into module.h in the future. If we don't do this, we'd
have to add module.h to obviously non-modular code, and that
would be a worse thing.

Note that direct use of __initcall is discouraged, vs. one
of the priority categorized subgroups. As __initcall gets
mapped onto device_initcall, our use of device_initcall
directly in this change means that the runtime impact is
zero -- it will remain at level 6 in initcall ordering.

Cc: Colin Cross <[e-mail]>
Cc: Jesse Barker <[e-mail]>
Cc: Android Kernel Team <[e-mail]>
Cc: John Stultz <[e-mail]>
Cc: Greg Kroah-Hartman <[e-mail]>
Signed-off-by: Paul Gortmaker <[e-mail]>
Signed-off-by: Greg Kroah-Hartman <[e-mail]>

drivers/staging/android/ion/ion_dummy_driver.c

···
 #include <linux/err.h>
 #include <linux/platform_device.h>
 #include <linux/slab.h>
+#include <linux/init.h>
 #include <linux/bootmem.h>
 #include <linux/memblock.h>
 #include <linux/sizes.h>
···
 	}
 	return err;
 }
+device_initcall(ion_dummy_init);
 
 static void __exit ion_dummy_exit(void)
 {
···
 
 	return;
 }
-
-module_init(ion_dummy_init);
-module_exit(ion_dummy_exit);
-
+__exitcall(ion_dummy_exit);