Update HomematicDeviceDiscoveryService.java (#11971)

Update HomematicDeviceDiscoveryService.java (#11971)

fix for bug #11969

After a restart of openHAB the function 'getScanTimeout()' is called before the member 'installModeDuration' has been initialized with the correct value from the configuration. With a large number of Homematic devices, the default value of 'installModeDuration' is too small to read in all Homematic devices from the CCU.
Therefore, when calling the function 'getScanTimeout()', the value is read directly from the configuration. The member 'installModeDuration' has been removed.

Signed-off-by: raykleibelt <54982000+raykleibelt@users.noreply.github.com>