Cover: fix position update

[farmio]

Description

when a cover can't move (eg. locked) and set_down() is called the travelcalculator starts. because of _position_current.value has not changed in this case its callback is not called when a GroupValueResponse comes in - eg. sync() is used.

Fixes # (issue)

Type of change

• Bug fix (non-breaking change which fixes an issue)
• New feature (non-breaking change which adds functionality)
• Breaking change (fix or feature that would cause existing functionality to not work as expected)
• This change requires a documentation update

Checklist:

• I have performed a self-review of my own code
• The documentation has been adjusted accordingly
• The changes generate no new warnings
• Tests have been added that prove the fix is effective or that the feature works
• The changes are documented in the changelog
• The Homeassistant plugin has been adjusted in case of new config options

[coveralls]

Coverage decreased (-0.1%) to 94.65% when pulling 1688395 on farmio:fix-cover-position-update into 30720c7 on XKNX:master.