hw/gpio/omap_gpio: Add local variable to avoid embedded cast

Add a local 'struct omap_gpif_s *' variable to improve readability.
(This also eases next commit conversion).

Signed-off-by: Philippe Mathieu-Daudé <philmd@linaro.org>
Reviewed-by: Richard Henderson <richard.henderson@linaro.org>
Message-id: 20230109140306.23161-3-philmd@linaro.org
Signed-off-by: Peter Maydell <peter.maydell@linaro.org>

hw/gpio/omap_gpio.c

@@ -53,7 +53,8 @@ struct omap_gpif_s {
 /* General-Purpose I/O of OMAP1 */
 static void omap_gpio_set(void *opaque, int line, int level)
 {
-    struct omap_gpio_s *s = &((struct omap_gpif_s *) opaque)->omap1;
+    struct omap_gpif_s *p = opaque;
+    struct omap_gpio_s *s = &p->omap1;
     uint16_t prev = s->inputs;
 
     if (level)