Devices with a type-cover have an additional "book" mode, deactivating
type-cover input and turning off its backlight. This is currently
unsupported, leading to the warning

  surface_aggregator_tablet_mode_switch 01:26:01:00:01: unknown device posture for type-cover: 6

Therefore, add support for this state and map it to enable tablet-mode.

Fixes: 37ff64cd ("platform/surface: aggregator_tabletsw: Add support for Type-Cover posture source")
Signed-off-by: Maximilian Luz <luzmaximilian@gmail.com>
Link: https://lore.kernel.org/r/20230525213218.2797480-3-luzmaximilian@gmail.comReviewed-by: Hans de Goede <hdegoede@redhat.com>
Signed-off-by: Hans de Goede <hdegoede@redhat.com>