Fix warning reported by Clang static code analyzer:

    CC      hw/timer/stm32f2xx_timer.o
  hw/timer/stm32f2xx_timer.c:225:9: warning: Value stored to 'value' is never read
          value = timer_val;
          ^       ~~~~~~~~~

Reported-by: Clang Static Analyzer
Reviewed-by: Alistair Francis <alistair.francis@wdc.com>
Signed-off-by: Philippe Mathieu-Daudé <philmd@redhat.com>
Message-Id: <20200422133152.16770-9-philmd@redhat.com>
Signed-off-by: Laurent Vivier <laurent@vivier.eu>