arch/x86: Use ssize_t to store length

size_t is the wrong type to store the return value of
acpi_device_path_fill(), since any negative error values will be
converted to a very large unsigned integer and potentially cause
buffer overflow.

Change-Id: Ia8ed62ecfac8eaa18a61545bd203b3c7a7cd9ca5
Signed-off-by: Jacob Garber <jgarber1@ualberta.ca>
Found-by: Coverity CID 1402095
Reviewed-on: https://review.coreboot.org/c/coreboot/+/33962
Tested-by: build bot (Jenkins) <no-reply@coreboot.org>
Reviewed-by: HAOUAS Elyes <ehaouas@noos.fr>
Reviewed-by: Paul Menzel <paulepanter@users.sourceforge.net>

src/arch/x86/acpi_device.c

@@ -148,7 +148,7 @@ const char *acpi_device_scope(struct device *dev)
 const char *acpi_device_path_join(struct device *dev, const char *name)
 {
 	static char buf[DEVICE_PATH_MAX] = {};
-	size_t len;
+	ssize_t len;
 
 	if (!dev)
 		return NULL;