bcma: use module_init for the main part of bus initialization

So far we were using fs_initcall. It was (and still is) needed because
struct bus_type has to be registered early. However main bus
initialization has to happen later as it requires SPROM which depends on
NVRAM which depends on mtd.
Solve it by using fs_initcall only for bus_register call and module_init
for the rest. It affects bcma only when built-in obviously.

This was tested with BCM4706 and BCM5357C0 (BCM47XX), BCM4708A0
(ARCH_BCM_5301X) and BCM43225 (PCIe card with bcma as module).

Signed-off-by: Rafał Miłecki <zajec5@gmail.com>
Signed-off-by: Kalle Valo <kvalo@codeaurora.org>