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: