seat-native: Fix has_touchscreen becoming FALSE when a non touchscreen device gets added
Unconditionally setting has_touchscreen to check_touch_mode when a new device gets added leads to has_touchscreen becoming false when during runtime e.g. an USB keyboard gets plugged in.
Fix this by setting has_touchscreen to TRUE when check_touch_mode is TRUE and leaving it alone otherwise.
!1506 (merged) (cherry picked from commit 6c240dc8)