Fix the return value of Read by Group response encoder
Fix the return value of Read by Group response encoder

To mantain consistency, the size (in bytes) written to the PDU should
be returned.

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