drm/imx/dcss: fix resource size calculation

The resource is inclusive of the ->start and ->end addresses so this
calculation is not correct.  It should be "res->end - res->start + 1".
Use the resource_size() to do the calculation.

Fixes: 90393c9 ("drm/imx/dcss: request memory region")
Signed-off-by: Dan Carpenter <dan.carpenter@linaro.org>
Reviewed-by: Laurentiu Palcu <laurentiu.palcu@oss.nxp.com>
Signed-off-by: Laurentiu Palcu <laurentiu.palcu@oss.nxp.com>
Link: https://patchwork.freedesktop.org/patch/msgid/4914592b-4256-4c9c-bc1d-6dec1e473831@moroto.mountain

Author: Dan Carpenter

drivers/gpu/drm/imx/dcss/dcss-dev.c

@@ -167,7 +167,6 @@ struct dcss_dev *dcss_dev_create(struct device *dev, bool hdmi_output)
 	struct resource *res;
 	struct dcss_dev *dcss;
 	const struct dcss_type_data *devtype;
-	resource_size_t res_len;
 
 	devtype = of_device_get_match_data(dev);
 	if (!devtype) {
@@ -181,8 +180,7 @@ struct dcss_dev *dcss_dev_create(struct device *dev, bool hdmi_output)
 		return ERR_PTR(-EINVAL);
 	}
 
-	res_len = res->end - res->start;
-	if (!devm_request_mem_region(dev, res->start, res_len, "dcss")) {
+	if (!devm_request_mem_region(dev, res->start, resource_size(res), "dcss")) {
 		dev_err(dev, "cannot request memory region\n");
 		return ERR_PTR(-EBUSY);
 	}