drivers: misc: ft8xx: constify device config access

`const` was missing from one device config access.

Signed-off-by: Gerard Marull-Paretas <gerard.marull@nordicsemi.no>
diff --git a/drivers/misc/ft8xx/ft8xx.c b/drivers/misc/ft8xx/ft8xx.c
index 7a3fc07..914a56e 100644
--- a/drivers/misc/ft8xx/ft8xx.c
+++ b/drivers/misc/ft8xx/ft8xx.c
@@ -96,7 +96,7 @@
 static int ft8xx_init(const struct device *dev)
 {
 	int ret;
-	struct ft8xx_config *config = (struct ft8xx_config *)dev->config;
+	const struct ft8xx_config *config = (const struct ft8xx_config *)dev->config;
 
 	ret = ft8xx_drv_init();
 	if (ret < 0) {