samples: mcumgr: smp_svr: increase dfu throughput over Bluetooth

Adjusted Bluetooth parameters that have an impact on MTU and DLE
negotiation during a connection.

Enabled the Packet Reassembly feature from mcumgr libraries in the SMP
Server sample with the Bluetooth transport configuration. This change
should increase the overall throughput of the DFU process over
Bluetooth.

Signed-off-by: Kamil Piszczek <Kamil.Piszczek@nordicsemi.no>