nb/intel/haswell/pcie.c: Avoid needless death

Using `config_of(dev)` to access `dev->chip_info` will make coreboot die
if the latter is NULL, which is the case for devices detected at runtime
(i.e. not statically declared in the devicetree). Given that the code is
designed to work when the PEG config is all-zeroes (devicetree default),
dying because `dev->chip_info` is NULL is foolish and unwarranted.

Introduce a helper function that returns a pointer to devicetree config
when available, and otherwise returns a pointer to a zero-filled static
struct. In addition, avoid an out-of-bounds access in the very unlikely
case where the device's function is too large.

Tested on Asrock B85M Pro4, can now boot when `device pci 01.0 on end`
is commented out in its devicetree. Without this commit, it could not.

Change-Id: Ia2d3a03da9eab601fb834b0c51a8a51c9ae14c33
Signed-off-by: Angel Pons <th3fanbus@gmail.com>
Reviewed-on: https://review.coreboot.org/c/coreboot/+/55690
Tested-by: build bot (Jenkins) <no-reply@coreboot.org>
Reviewed-by: Nico Huber <nico.h@gmx.de>

src/northbridge/intel/haswell/pcie.c

@@ -46,15 +46,28 @@ static const char *pcie_acpi_name(const struct device *dev)
 }
 #endif
 
+static const struct peg_config *get_peg_config(struct device *dev, const uint8_t func)
+{
+	static const struct peg_config default_config = { 0 };
+
+	if (!dev || !dev->chip_info)
+		return &default_config;
+
+	const struct northbridge_intel_haswell_config *config = dev->chip_info;
+
+	if (func >= ARRAY_SIZE(config->peg_cfg)) {
+		printk(BIOS_ERR, "%s: Found PEG function %u, which doesn't exist on Haswell\n",
+		       __func__, func);
+		return &default_config;
+	}
+	return &config->peg_cfg[func];
+}
+
 static void peg_enable(struct device *dev)
 {
-	const struct northbridge_intel_haswell_config *config = config_of(dev);
-
 	const uint8_t func = PCI_FUNC(dev->path.pci.devfn);
 
-	assert(func < ARRAY_SIZE(config->peg_cfg));
-
-	const struct peg_config *peg_cfg = &config->peg_cfg[func];
+	const struct peg_config *peg_cfg = get_peg_config(dev, func);
 
 	const bool slot_implemented = !peg_cfg->is_onboard;