Lefant N3: rename work_mode back to command

I had some back and forwards about including the switch_charge dp
in with commands, and which neeeded to be the primary, then forgot to
rename work_mode back to command when I returned back to that as primary.

Mark dp 2 as optional to help detection.

Issue #1393

custom_components/tuya_local/devices/lefant_n3_vacuum.yaml

@@ -13,6 +13,7 @@ primary_entity:
     - id: 2
       name: activate
       type: boolean
+      optional: true
       mapping:
         - dps_val: true
           value: false
@@ -24,7 +25,7 @@ primary_entity:
       optional: true
       hidden: true
     - id: 4
-      name: work_mode
+      name: command
       type: string
       optional: true
       mapping: