Bluetooth: Host: Call att_mtu_updated callback on channel reconfigured

Previously, the att_mtu_updated callback was only called on initial
connection of the channel or during the MTU Exchange procedure. There
was no way for the application to know that the MTU increased in the
case where the peer initiated the reconfiguration.

Signed-off-by: Herman Berget <herman.berget@nordicsemi.no>