os: Recompute whether any clients are ready after check_timers()

If a driver calls AttendClient() from within a timer callback we
need to re-compute the local 'are_ready' to prevent the attended
client from waiting until WaitForSomething() times out.

This is a fix similar to commit 9ed5b263.

Signed-off-by: Damien Leone <dleone@nvidia.com>
Reviewed-by: Keith Packard <keithp@keithp.com>