PCI: host-generic: Fix NULL pointer dereference on 32-bit CAM systems

In the Linux kernel, the following vulnerability has been resolved:

PCI: host-generic: Fix NULL pointer dereference on 32-bit CAM systems

On 32-bit systems the config space is too large to ioremap in one go, so
pci_ecam_create() maps each bus segment separately and relies on the
->add_bus callback (pci_ecam_add_bus) to populate the per-bus mapping in
cfg->winp[]. pci_ecam_map_bus() then uses that mapping as the base for
every config access.

The generic ECAM ops (pci_generic_ecam_ops) already provide the ->add_bus
and ->remove_bus callbacks, but the CAM (legacy) ops in pci-host-generic.c
do not. As a result, on a 32-bit host using "pci-host-cam-generic" the
per-bus mapping is never set up and the first config read dereferences a
NULL base, crashing during bus enumeration:

Unable to handle kernel NULL pointer dereference at virtual address 00000800
Oops [#1]
CPU: 0 PID: 1 Comm: swapper Not tainted 6.9.7+ #43
Hardware name: Digilent Nexys-Video-A7 RV32 (DT)
epc : pci_generic_config_read+0x40/0xb0
ra : pci_generic_config_read+0x2c/0xb0
[<c038db9c>] pci_generic_config_read+0x40/0xb0
[<c038da04>] pci_bus_read_config_dword+0x50/0xb0
[<c0391e94>] pci_bus_generic_read_dev_vendor_id+0x3c/0x1ec
[<c039245c>] pci_scan_single_device+0xa4/0x11c
[<c0392570>] pci_scan_slot+0x9c/0x23c
[<c039388c>] pci_scan_child_bus_extend+0x58/0x2f4
[<c0393db0>] pci_scan_root_bus_bridge+0x64/0xe8
[<c0393e54>] pci_host_probe+0x20/0xc8
[<c03bc6f4>] pci_host_common_probe+0x144/0x1e4

Fix this by giving the CAM ops the same ->add_bus/->remove_bus callbacks.
Since pci_ecam_add_bus() and pci_ecam_remove_bus() are static to ecam.c,
move the CAM ops definition there as pci_generic_cam_ops (mirroring
pci_generic_ecam_ops) and export it for pci-host-generic.c to reference.

[mani: removed timestamp from log]