libpayload: Add UNKNOWN_SPEED to usb_speed enum

xhci_rh_port_speed return -1 if the port is disabled. The usb_speed enum
is unsigned so this results in a positive value which implies success.

Adding a -1 to the enum will make it signed so the >= 0 check will work
correctly.

BUG=b:76831439
TEST=verified on grunt that -1 is returned when port is disabled.

Change-Id: I98a373717d52dfb6ca4dcc53a00dc1b4c240a919
Signed-off-by: Raul E Rangel <rrangel@chromium.org>
Reviewed-on: https://review.coreboot.org/27476
Tested-by: build bot (Jenkins) <no-reply@coreboot.org>
Reviewed-by: Julius Werner <jwerner@chromium.org>

payloads/libpayload/drivers/usb/xhci_rh.c

@@ -95,7 +95,7 @@ xhci_rh_port_speed(usbdev_t *const dev, const int port)
 				>> PORTSC_PORT_SPEED_START)
 			- 1;
 	} else {
-		return -1;
+		return UNKNOWN_SPEED;
 	}
 }
 

payloads/libpayload/include/usb/usb.h

@@ -187,7 +187,11 @@ typedef struct {
 } endpoint_t;
 
 typedef enum {
-	FULL_SPEED = 0, LOW_SPEED = 1, HIGH_SPEED = 2, SUPER_SPEED = 3,
+	UNKNOWN_SPEED = -1,
+	FULL_SPEED = 0,
+	LOW_SPEED = 1,
+	HIGH_SPEED = 2,
+	SUPER_SPEED = 3,
 } usb_speed;
 
 struct usbdev {