misc: alcor_pci: fix null-ptr-deref when there is no PCI bridge

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

misc: alcor_pci: fix null-ptr-deref when there is no PCI bridge

There is an issue with the ASPM(optional) capability checking function.

A device might be attached to root complex directly, in this case,

bus->self(bridge) will be NULL, thus priv->parent_pdev is NULL.

Since alcor_pci_init_check_aspm(priv->parent_pdev) checks the PCI link's

ASPM capability and populate parent_cap_off, which will be used later by

alcor_pci_aspm_ctrl() to dynamically turn on/off device, what we can do

here is to avoid checking the capability if we are on the root complex.

This will make pdev_cap_off 0 and alcor_pci_aspm_ctrl() will simply

return when bring called, effectively disable ASPM for the device.

[ 1.246492] BUG: kernel NULL pointer dereference, address: 00000000000000c0

[ 1.248731] RIP: 0010:pci_read_config_byte+0x5/0x40

[ 1.253998] Call Trace:

[ 1.254131] ? alcor_pci_find_cap_offset.isra.0+0x3a/0x100 [alcor_pci]

[ 1.254476] alcor_pci_probe+0x169/0x2d5 [alcor_pci]