platform/x86: lenovo-yogabook: Add YB_KBD_BL_MAX define

Add a define for the max brightness level instead of hardcoding
this to 255 in multiple places.

Signed-off-by: Hans de Goede <hdegoede@redhat.com>
Link: https://lore.kernel.org/r/20230430165807.472798-17-hdegoede@redhat.com