[homematic] Fix updating enum config values (#14213)

[homematic] Fix updating enum config values (#14213)

When changing an enum value in the configuration, we used the wrong data
type: while the value in the OH config is a string (the 'option value' -
see HomematicThingHandler::getValueForConfiguration), internally we use
an integer (the 'option index'), so we have to do the option value ->
option index conversion when applying the new value.
This especially was a problem for HM-MOD-EM-8 devices, which check the
CHANNEL_FUNCTION enum value as part of their initialization routine.
When disabling/enabling them after changing the CHANNEL_FUNCTION enum
value, they went offline, because their initialization failed due to a
NumberFormatException (via
HomematicThingHandler::doInitializeInBackground ->
HmChannel::checkForChannelFunctionChange ->
HmChannel::getCurrentFunction)

Signed-off-by: Danny Baumann <dannybaumann@web.de>