drivers: counter: Fix unchecked return value in mcp7940

Fix unchecked return value scanned by Coverity.

(cherry picked from commit 364c555)

Original-Signed-off-by: James Roy <rruuaanng@outlook.com>
GitOrigin-RevId: 364c555
Cr-Build-Id: 8726861018524474785
Cr-Build-Url: https://cr-buildbucket.appspot.com/build/8726861018524474785
Copybot-Job-Name: zephyr-main-copybot-downstream
Change-Id: I8fca958b4de59b2940edcd749ecfb032349f7a1e
Reviewed-on: https://chromium-review.googlesource.com/c/chromiumos/third_party/zephyr/+/6138314
Commit-Queue: ChromeOS Prod (Robot) <chromeos-ci-prod@chromeos-bot.iam.gserviceaccount.com>
Tested-by: ChromeOS Prod (Robot) <chromeos-ci-prod@chromeos-bot.iam.gserviceaccount.com>
Bot-Commit: ChromeOS Prod (Robot) <chromeos-ci-prod@chromeos-bot.iam.gserviceaccount.com>

drivers/counter/rtc_mcp7940n.c

@@ -712,7 +712,7 @@ static int mcp7940n_init(const struct device *dev)
 		gpio_init_callback(&data->int_callback, mcp7940n_init_cb,
 				   BIT(cfg->int_gpios.pin));
 
-		gpio_add_callback(cfg->int_gpios.port, &data->int_callback);
+		(void)gpio_add_callback(cfg->int_gpios.port, &data->int_callback);
 
 		/* Configure interrupt polarity */
 		if ((cfg->int_gpios.dt_flags & GPIO_ACTIVE_LOW) == GPIO_ACTIVE_LOW) {