x86: timer: Use a separate flag for whether timer is inited

At present the value of the timer base is used to determine whether the
timer has been set up or not. It is true that the timer is essentially
never exactly 0 when it is read. However 'time 0' may indicate the time
that the machine was reset so it is useful to be able to denote that.

Update the code to use a separate flag instead.

Signed-off-by: Simon Glass <sjg@chromium.org>
Tested-by: Aiden Park <aiden.park@intel.com>
Reviewed-by: Aiden Park <aiden.park@intel.com>
Reviewed-by: Bin Meng <bmeng.cn@gmail.com>