x86: tsc_timer: Correct overflow in __udelay()

At present long delays such as msleep(2000) can cause an overflow in this
function. There is no need for this, since it already uses a 64-bit int.

Add a cast to correct this.

Signed-off-by: Simon Glass <sjg@chromium.org>
Reviewed-by: Bin Meng <bmeng.cn@gmail.com>
diff --git a/drivers/timer/tsc_timer.c b/drivers/timer/tsc_timer.c
index 706d52b..7d0fc66 100644
--- a/drivers/timer/tsc_timer.c
+++ b/drivers/timer/tsc_timer.c
@@ -372,7 +372,7 @@
 	u64 now = get_ticks();
 	u64 stop;
 
-	stop = now + usec * get_tbclk_mhz();
+	stop = now + (u64)usec * get_tbclk_mhz();
 
 	while ((int64_t)(stop - get_ticks()) > 0)
 #if defined(CONFIG_QEMU) && defined(CONFIG_SMP)