Fix error during setup process of Mikrotik

[springstan]

Description:

• this PR solves an error during setup of a Mikrotik hub
• example error:

2020-01-15 21:25:42 ERROR (MainThread) [homeassistant.setup] Error during setup of component mikrotik
Traceback (most recent call last):
  File "/srv/homeassistant/lib/python3.7/site-packages/homeassistant/setup.py", line 176, in _async_setup_component
    component.setup, hass, processed_config  # type: ignore
  File "/usr/lib/python3.7/concurrent/futures/thread.py", line 57, in run
    result = self.fn(*self.args, **self.kwargs)
  File "/srv/homeassistant/lib/python3.7/site-packages/homeassistant/components/mikrotik/__init__.py", line 94, in setup
    api.connect_to_device()
  File "/srv/homeassistant/lib/python3.7/site-packages/homeassistant/components/mikrotik/__init__.py", line 153, in connect_to_device
    self.hostname = self.get_hostname()
  File "/srv/homeassistant/lib/python3.7/site-packages/homeassistant/components/mikrotik/__init__.py", line 160, in get_hostname
    return data[0][NAME] if data else None
TypeError: 'generator' object is not subscriptable

Related issue (if applicable): fixes #30745
Related PR (if applicable): completes #30800

Example entry for configuration.yaml (if applicable):

mikrotik:
  - host: 192.168.1.1
    username: secret
    password: secret
    login_method: plain
    port: 8728

Checklist:

• The code change is tested and works locally. (see Mikrotik hub is broken? #30745 (comment))
• Local tests pass with tox. Your PR cannot be merged unless tests pass
• There is no commented out code in this PR.
• I have followed the development checklist