Fix interval interrupt of cadence ttc when timer is in decrement mode

The interval interrupt is not set if the timer is in decrement mode.
This is because x >=0 and x < interval after leaving the while-loop.

Signed-off-by: Johannes Schlatow <schlatow@ida.ing.tu-bs.de>
Message-id: 20150630135821.51f3b4fd@johanness-latitude
Reviewed-by: Peter Crosthwaite <peter.crosthwaite@xilinx.com>
Signed-off-by: Peter Maydell <peter.maydell@linaro.org>
