libpayload: usb: handle situation with no free device address

Change-Id: I1308bdca90f1a09d980f384ee85552198a39b965
Signed-off-by: Patrick Georgi <pgeorgi@chromium.org>
Found-by: Coverity Scan #1260940
Reviewed-on: https://review.coreboot.org/18036
Tested-by: build bot (Jenkins)
Reviewed-by: Martin Roth <martinroth@google.com>

payloads/libpayload/drivers/usb/usb.c

@@ -336,6 +336,8 @@ generic_set_address (hci_t *controller, usb_speed speed,
 		     int hubport, int hubaddr)
 {
 	int adr = get_free_address (controller);	// address to set
+	if (adr < 0)
+		return NULL;
 	dev_req_t dr;
 
 	memset (&dr, 0, sizeof (dr));