bcma: fix build error when build as module

Currently of_default_bus_match_table is not exported so we can only use
this feature when bcma is build into the kernel. This patch removes
support for child buses when bcma is build as a module as a temporary
fix for a build problem introduces in this commit:

commit cae761b5
Author: Rafał Miłecki <zajec5@gmail.com>
Date:   Sun Jun 28 17:17:13 2015 +0200

    bcma: populate bus DT subnodes as platform_device-s

Reported-by: Stephen Rothwell <sfr@canb.auug.org.au>
Fixes: cae761b5 ("bcma: populate bus DT subnodes as platform_device-s")
Signed-off-by: Hauke Mehrtens <hauke@hauke-m.de>
Signed-off-by: Kalle Valo <kvalo@codeaurora.org>