core: Modbus Switch toggle doesn't work with latest code

The problem

With latest (nightly) Modbus code I am no longer able to toggle the Modbus switch.

I believe the error is related to PR #49910. I will investigate further whether I have a non-standard configuration, or if this is really an issue with our Modbus integration.

What is version of Home Assistant Core has the issue?

core-2021.6.0.dev0

What was the last working version of Home Assistant Core?

No response

What type of installation are you running?

Home Assistant Core

Integration causing the issue

Modbus

Link to integration documentation on our website

https://www.home-assistant.io/integrations/modbus/

Example YAML snippet

modbus:
  - name: plc_01
    type: tcp
    host: 127.0.0.1
    port: 5020

    switches:
      - name: plc_01_in_switch
        address: 8
        write_type: coil
        slave: 1
        scan_interval: 1

Anything in the logs that might be useful for us?

2021-05-03 11:30:19 ERROR (SyncWorker_0) [homeassistant.components.modbus.modbus] Pymodbus: WriteCoilResponse(8) => 1

Additional information

No response