drm/udl: The number of pixels is always positive

Store sku_pixel_limit as type unsigned long instead of int. The
number of pixels available is always positive.

Signed-off-by: Thomas Zimmermann <[e-mail]>
Reviewed-by: Patrik Jakobsson <[e-mail]>
Link: https://lore.kernel.org/r/[e-mail]

drivers/gpu/drm/udl/udl_drv.h

···
 struct udl_device {
 	struct drm_device drm;
 
+	unsigned long sku_pixel_limit;
+
 	struct drm_plane primary_plane;
 	struct drm_crtc crtc;
 	struct drm_encoder encoder;
 	struct drm_connector connector;
-
-	int sku_pixel_limit;
 
 	struct urb_list urbs;
 };