bcma: fix unregistration of cores

When cores are unregistered, entries
need to be removed from cores list in a safe manner.

Reported-by: Stanislaw Gruszka <sgruszka@redhat.com>
Reviewed-by: Arend Van Spriel <arend@broadcom.com>
Signed-off-by: Piotr Haber <phaber@broadcom.com>
Cc: stable@vger.kernel.org
Signed-off-by: John W. Linville <linville@tuxdriver.com>