platform/surface: aggregator_tabletsw: Add support for book mode in POS subsystem

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>

drivers/platform/surface/surface_aggregator_tabletsw.c

@@ -340,6 +340,7 @@ enum ssam_pos_state_cover {
 	SSAM_POS_COVER_LAPTOP        = 0x03,
 	SSAM_POS_COVER_FOLDED_CANVAS = 0x04,
 	SSAM_POS_COVER_FOLDED_BACK   = 0x05,
+	SSAM_POS_COVER_BOOK          = 0x06,
 };
 
 enum ssam_pos_state_sls {
@@ -372,6 +373,9 @@ static const char *ssam_pos_state_name_cover(struct ssam_tablet_sw *sw, u32 stat
 	case SSAM_POS_COVER_FOLDED_BACK:
 		return "folded-back";
 
+	case SSAM_POS_COVER_BOOK:
+		return "book";
+
 	default:
 		dev_warn(&sw->sdev->dev, "unknown device posture for type-cover: %u\n", state);
 		return "<unknown>";
@@ -421,6 +425,7 @@ static bool ssam_pos_state_is_tablet_mode_cover(struct ssam_tablet_sw *sw, u32 s
 	case SSAM_POS_COVER_DISCONNECTED:
 	case SSAM_POS_COVER_FOLDED_CANVAS:
 	case SSAM_POS_COVER_FOLDED_BACK:
+	case SSAM_POS_COVER_BOOK:
 		return true;
 
 	case SSAM_POS_COVER_CLOSED: