hw/gpio/aspeed_gpio: Remove dead assignment

hw/gpio/aspeed_gpio: Remove dead assignment

Fix warning reported by Clang static code analyzer:

  hw/gpio/aspeed_gpio.c:717:18: warning: Value stored to 'g_idx' during its initialization is never read
      int set_idx, g_idx = *group_idx;
                   ^~~~~   ~~~~~~~~~~

Reported-by: Clang Static Analyzer
Reviewed-by: Cédric Le Goater <clg@kaod.org>
Signed-off-by: Philippe Mathieu-Daudé <philmd@redhat.com>
Message-Id: <20200422133152.16770-8-philmd@redhat.com>
Signed-off-by: Laurent Vivier <laurent@vivier.eu>