att: Replace ATT_MAX_MTU with ATT_MAX_VALUE_LEN
att: Replace ATT_MAX_MTU with ATT_MAX_VALUE_LEN

ATT has the concept that an attribute value has a maximum length and we
weren't keeping track of this.

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