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>