PCI: Consolidate "next-function" functions

There are several next_fn functions (no_next_fn, next_trad_fn,
next_ari_fn); consolidate them in next_fn() to simplify the code.

[bhelgaas: make next_fn() static, rework control flow]
Signed-off-by: Yijing Wang <wangyijing@huawei.com>
Signed-off-by: Jiang Liu <jiang.liu@huawei.com>
Signed-off-by: Bjorn Helgaas <bhelgaas@google.com>