audio/AVRCP: Fix invalid response to RegisterNotification
audio/AVRCP: Fix invalid response to RegisterNotification

The response to RegisterNotification for event settings changed was
not setting the initial length properly which cause the code to send
malformed/invalid PDUs.

ComputeCycles/bluez,pkarasev3/bluez,ComputeCycles/bluez,pkarasev3/bluez,mapfau/bluez,silent-snowman/bluez,pstglia/external-bluetooth-bluez,pkarasev3/bluez,pstglia/external-bluetooth-bluez,silent-snowman/bluez,pstglia/external-bluetooth-bluez,mapfau/bluez,pkarasev3/bluez,silent-snowman/bluez,pstglia/external-bluetooth-bluez,ComputeCycles/bluez,mapfau/bluez,mapfau/bluez,silent-snowman/bluez,ComputeCycles/bluez