mn10300: don't use module_init in non-modular flash.c code

The flash.o is built for obj-y -- and hence this code is always
present.  It will never be modular, so using module_init as an alias
for __initcall can be somewhat 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.

Reported-by: kbuild test robot <fengguang.wu@intel.com>
Cc: David Howells <dhowells@redhat.com>
Acked-by: David Howells <dhowells@redhat.com>
Cc: Koichi Yasutake <yasutake.koichi@jp.panasonic.com>
Cc: linux-am33-list@redhat.com
Signed-off-by: Paul Gortmaker <paul.gortmaker@windriver.com>