mb/emulation/qemu-i440fx: Refactor `fw_cfg_max_cpus()`

Return 0 instead of -1 in case of error. Both values indicate an error
has happened. Adapt `cpu_bus_scan()` accordingly.

Change-Id: I0f83fdc41c20ed3aae80829432fc84024f5b9b47
Signed-off-by: Angel Pons <th3fanbus@gmail.com>
Reviewed-on: https://review.coreboot.org/c/coreboot/+/58918
Tested-by: build bot (Jenkins) <no-reply@coreboot.org>
Reviewed-by: Felix Held <felix-coreboot@felixheld.de>
diff --git a/src/mainboard/emulation/qemu-i440fx/fw_cfg.c b/src/mainboard/emulation/qemu-i440fx/fw_cfg.c
index 01be2c3..3206e4c 100644
--- a/src/mainboard/emulation/qemu-i440fx/fw_cfg.c
+++ b/src/mainboard/emulation/qemu-i440fx/fw_cfg.c
@@ -133,7 +133,7 @@
 	unsigned short max_cpus;
 
 	if (!fw_cfg_present())
-		return -1;
+		return 0;
 
 	fw_cfg_get(FW_CFG_MAX_CPUS, &max_cpus, sizeof(max_cpus));
 	return max_cpus;
diff --git a/src/mainboard/emulation/qemu-i440fx/northbridge.c b/src/mainboard/emulation/qemu-i440fx/northbridge.c
index d041b0c..9785a3a 100644
--- a/src/mainboard/emulation/qemu-i440fx/northbridge.c
+++ b/src/mainboard/emulation/qemu-i440fx/northbridge.c
@@ -265,11 +265,11 @@
 
 static void cpu_bus_scan(struct device *bus)
 {
-	int max_cpus = fw_cfg_max_cpus();
+	unsigned int max_cpus = fw_cfg_max_cpus();
 	struct device *cpu;
 	int i;
 
-	if (max_cpus < 0)
+	if (max_cpus == 0)
 		return;
 	/*
 	 * Do not install more CPUs than supported by coreboot.