Use timeval helpers

Some of these changes actually fix broken comparisons which could
occasionally fail. Others just clean up the code and make it more clear.

Change-Id: I6c398bdc45fa0d2716f48a74822457d1351f81a5
Signed-off-by: Christopher Head <chead@zaber.com>
Reviewed-on: http://openocd.zylin.com/4380
Tested-by: jenkins
Reviewed-by: Tomas Vanek <vanekt@fbl.cz>

src/jtag/drivers/presto.c

@@ -117,8 +117,7 @@ static int presto_read(uint8_t *buf, uint32_t size)
 		ftbytes += presto->retval;
 
 		gettimeofday(&now, NULL);
-		if ((now.tv_sec > timeout.tv_sec) ||
-				((now.tv_sec == timeout.tv_sec) && (now.tv_usec > timeout.tv_usec)))
+		if (timeval_compare(&now, &timeout) > 0)
 			break;
 	}