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 <tzimmermann@suse.de>
Reviewed-by: Patrik Jakobsson <patrik.r.jakobsson@gmail.com>
Link: https://lore.kernel.org/r/20250410105948.25463-5-tzimmermann@suse.de