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 <ccross@android.com>
Cc: Jesse Barker <jesse.barker@arm.com>
Cc: Android Kernel Team <kernel-team@android.com>
Cc: John Stultz <john.stultz@linaro.org>
Cc: Greg Kroah-Hartman <gregkh@linuxfoundation.org>
Signed-off-by: Paul Gortmaker <paul.gortmaker@windriver.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>

drivers/staging/android/ion/ion_dummy_driver.c

@@ -17,6 +17,7 @@
 #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>
@@ -129,6 +130,7 @@ err:
 	}
 	return err;
 }
+device_initcall(ion_dummy_init);
 
 static void __exit ion_dummy_exit(void)
 {
@@ -153,7 +155,4 @@ static void __exit ion_dummy_exit(void)
 
 	return;
 }
-
-module_init(ion_dummy_init);
-module_exit(ion_dummy_exit);
-
+__exitcall(ion_dummy_exit);