Make mikrotik method setting optional as intended

[wolph]

Pull request #17852 accidently made the method setting required.

Description:

Pull request #17852 accidently made the method setting required while it was intended to be optional. This tiny change fixes that :)

Related issue (if applicable):

Pull request in home-assistant.io with documentation (if applicable):

Example entry for configuration.yaml (if applicable):

device_tracker:
  - platform: mikrotik
    host: IP_ADDRESS
    username: ADMIN_USERNAME
    password: ADMIN_PASSWORD

Checklist:

• The code change is tested and works locally.
• Local tests pass with tox. Your PR cannot be merged unless tests pass
• There is no commented out code in this PR.