gattrib: Fix g_attrib_set_mtu
gattrib: Fix g_attrib_set_mtu

23 octets is the default (and minimum) ATT_MTU value. If someone tries
to set ATT_MTU less than 23 octets g_attrib_set_mtu should fail (return
FALSE). Additionally, there is no constraint regarding the maximum value
of ATT_MTU, so we should not check for it.

Also, we should not change the L2CAP ATT fixed channel MTU. bt_io_set
call will always fail since we are not supposed to change L2CAP MTU
after connection is established.

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