LinuxKPI: pci: fix pcie_get_speed_cap()

pcie_get_speed_cap() has a hard coded skip of 3 devices at the
beginning.  It is either called on a pdev or on a result from
pci_upstream_bridge().  In the latter case skipping another three
devices might get us to acpi0 or nexus, neither of which is a
PCI device still and pci_get_vendor() will panic() on that.

Sponsored by:	The FreeBSD Foundation (commit)
GHI:		https://github.com/freebsd/drm-kmod/issues/393
MFC after:	2 weeks
Differential Revision: https://reviews.freebsd.org/D53862

sys/compat/linuxkpi/common/include/linux/pci.h

@@ -1136,19 +1136,28 @@ pci_num_vf(struct pci_dev *dev)
 static inline enum pci_bus_speed
 pcie_get_speed_cap(struct pci_dev *dev)
 {
+	struct pci_dev *pbus;
 	device_t root;
 	uint32_t lnkcap, lnkcap2;
 	int error, pos;
 
-	root = device_get_parent(dev->dev.bsddev);
-	if (root == NULL)
-		return (PCI_SPEED_UNKNOWN);
-	root = device_get_parent(root);
-	if (root == NULL)
-		return (PCI_SPEED_UNKNOWN);
-	root = device_get_parent(root);
-	if (root == NULL)
-		return (PCI_SPEED_UNKNOWN);
+	/*
+	 * We should always be called on a PCI device.
+	 * The only current consumer I could find was amdgpu which either
+	 * calls us directly on a pdev(drmn?) or with the result of
+	 * pci_upstream_bridge().
+	 *
+	 * Treat "drmn" as special again as it is not a PCI device.
+	 */
+	if (dev->pdrv != NULL && dev->pdrv->isdrm) {
+		pbus = pci_upstream_bridge(dev);
+		if (pbus == NULL)
+			return (PCI_SPEED_UNKNOWN);
+	} else
+		pbus = dev;
+
+	/* "root" may be misleading as it may not be that. */
+	root = pbus->dev.bsddev;
 
 	if (pci_get_vendor(root) == PCI_VENDOR_ID_VIA ||
 	    pci_get_vendor(root) == PCI_VENDOR_ID_SERVERWORKS)