The function adf_cfg_add_key_value_param() allows to insert duplicates
entries in the key value store of the driver.

Change the behaviour of that function to the following policy:
- if the key doesn't exist, add it;
- if the key already exists with a different value, then delete it and
  replace it with a new one containing the new value;
- if the key exists with the same value, then return without doing
  anything.

The behaviour of this function has been changed in order to easily
update key-values in the driver database. In particular this is required
to update the value of the ServiceEnables key used to change the service
loaded on a device.
Signed-off-by: Giovanni Cabiddu <giovanni.cabiddu@intel.com>
Reviewed-by: Adam Guerin <adam.guerin@intel.com>
Reviewed-by: Fiona Trahe <fiona.trahe@intel.com>
Reviewed-by: Wojciech Ziemba <wojciech.ziemba@intel.com>
Reviewed-by: Vladis Dronov <vdronov@redhat.com>
Signed-off-by: Herbert Xu <herbert@gondor.apana.org.au>