Set default keyboard backlight brightness on probe(), this fixes
the backlight being off after a rmmod + modprobe.

Fixes: c0549b72 ("platform/x86: lenovo-yogabook-wmi: Add driver for Lenovo Yoga Book")
Signed-off-by: Hans de Goede <hdegoede@redhat.com>
Link: https://lore.kernel.org/r/20230430165807.472798-5-hdegoede@redhat.com