bcma: move code for core registration into separate function

This cleans code a bit and will us to register cores in other places as
well. The only difference with this patch is using "core_index" for
setting device name.

Signed-off-by: Rafał Miłecki <zajec5@gmail.com>
Acked-by: Hauke Mehrtens <hauke@hauke-m.de>
Signed-off-by: John W. Linville <linville@tuxdriver.com>