attrib-server: Fix gatt_channel MTU value
attrib-server: Fix gatt_channel MTU value

In attrib_channel_attach, channel->mtu should be initialized according
to ATT_MTU value.

Over BR/EDR, ATT_MTU should be set to the L2CAP imtu negotiated during
L2CAP configuration phase. Over LE, ATT_MTU should be 23 octets.

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