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]

Co-developed-by: Greg Kroah-Hartman <[e-mail]>
Signed-off-by: Tong Zhang <[e-mail]>
Link: https://lore.kernel.org/r/[e-mail]
Signed-off-by: Greg Kroah-Hartman <[e-mail]>

drivers/misc/cardreader/alcor_pci.c

···
 	u32 val32;
 
 	priv->pdev_cap_off    = alcor_pci_find_cap_offset(priv, priv->pdev);
-	priv->parent_cap_off = alcor_pci_find_cap_offset(priv,
+	/*
+	 * A device might be attached to root complex directly and
+	 * priv->parent_pdev will be NULL. In this case we don't check its
+	 * capability and disable ASPM completely.
+	 */
+	if (!priv->parent_pdev)
+		priv->parent_cap_off = alcor_pci_find_cap_offset(priv,
 							 priv->parent_pdev);
 
 	if ((priv->pdev_cap_off == 0) || (priv->parent_cap_off == 0)) {