Add wall clock timeout to mpsse_flush()

I think that libusb_handle_events_timeout_completed is supposed to make
progress or time out, but sometimes we hit a case where it makes no
progress, and mpsse_flush() loops forever. This wall clock timeout kicks
it out of that loop. OpenOCD appears to die afterwards, but that's still
an improvement.

Change-Id: Id9220557625834fb5b7dccf65251651a11a887f0

src/jtag/drivers/mpsse.c

@@ -892,6 +892,7 @@ int mpsse_flush(struct mpsse_ctx *ctx)
 	}
 
 	/* Polling loop, more or less taken from libftdi */
+	int64_t start = timeval_ms();
 	while (!write_result.done || !read_result.done) {
 		struct timeval timeout_usb;
 
@@ -914,6 +915,11 @@ int mpsse_flush(struct mpsse_ctx *ctx)
 					break;
 			}
 		}
+
+		if (timeval_ms() - start > 2000) {
+			LOG_ERROR("Timed out handling USB events in mpsse_flush().");
+			break;
+		}
 	}
 
 error_check: