Fix missing update of cached characteristic value

When we receive a characteristic update we have to update the cached
value on the Qt side. This was accidentally missed during an earlier
commit.

Change-Id: I0f84e35ee44e38d1e46b7ddcd4e78b7e216e49d5
Reviewed-by: Timur Pocheptsov <Timur.Pocheptsov@digia.com>
Reviewed-by: Alex Blasche <alexander.blasche@theqtcompany.com>