xtensa: don't use module_init for non-modular core network.c code

xtensa: don't use module_init for non-modular core network.c code

The network.c code is piggybacking off of the arch independent
CONFIG_NET, which is bool.  So the code is either built in 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.

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.   Should someone with real hardware
for boot testing want to change it later to arch_initcall or
something different, they can do that at a later date.

Cc: Chris Zankel <chris@zankel.net>
Cc: Max Filippov <jcmvbkbc@gmail.com>
Cc: Thomas Meyer <thomas@m3y3r.de>
Cc: linux-xtensa@linux-xtensa.org
Signed-off-by: Paul Gortmaker <paul.gortmaker@windriver.com>