BlueZ: Fix leaking client socket when running QLEController Peripheral mode

In fact there are two socket leaks. The first is the socket for the incoming
l2cp connection from the central device and the second one is allocated in the
ctor of QBluetoothSocket. When QBluetoothSocket::setSocketDescriptor is called
the previously ctor allocated socket was simply ignorred. This patch closes
both socket.

Fixes: QTBUG-75278
Change-Id: Ia483e3c2a04bec3a53ddf744c22b794941edf848
Reviewed-by: Timur Pocheptsov <timur.pocheptsov@qt.io>
Reviewed-by: Oliver Wolff <oliver.wolff@qt.io>