Bluetooth: SMP: Fix not allowing any PDU after pairing is finished

If pairing was finished it is perfectly fine for remote to send
another SMP PDU and start new pairing.

To make code simpler just use smp_reset() to set SMP into correct
state.

Change-Id: I8d04a5f0ed78c01ef4caca89f02a1ac5fe950e90
Signed-off-by: Szymon Janc <ext.szymon.janc@tieto.com>