MIPS: PCI: add controllers before the specified head

With commit 23dac14d ("MIPS: PCI: Use struct list_head lists") new
controllers are added after the specified head where they where added
before the specified head previously.

Use list_add_tail to restore the former order.

This patches fixes the following PCI error on lantiq:

  pci 0000:01:00.0: BAR 0: error updating (0x1c000004 != 0x000000)

Fixes: 23dac14d

 ("MIPS: PCI: Use struct list_head lists")
Signed-off-by: Mathias Kresin <dev@kresin.me>
Cc: linux-mips@linux-mips.org
Patchwork: https://patchwork.linux-mips.org/patch/15808/
Signed-off-by: Ralf Baechle <ralf@linux-mips.org>