bcma: clean bus initialization code

This moves main bus init code to the main.c and renames old function to
make its purpose clear.
Thanks to this change we'll also be able to separate scanning from
registration (and support PCIe Gen 2 devices) in the future.

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