MIPS: don't use module_init in non-modular cobalt/mtd.c file

As of commit 34b1252b ("MIPS:
Cobalt: Do not build MTD platform device registration code as module.")
this file became built-in instead of modular.  So we should also
stop using module_init as an alias for __initcall as that can be
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.

Direct use of __initcall is discouraged, vs prioritized ones.
Use of device_initcall is consistent with what __initcall
maps onto, and hence does not change the init order, making the
impact of this change zero.

Cc: Ralf Baechle <ralf@linux-mips.org>
Cc: linux-mips@linux-mips.org
Signed-off-by: Paul Gortmaker <paul.gortmaker@windriver.com>