Clear the m_open flag when closing PlutoSDR input and output
devices. Previously closeDevice() released m_deviceParams but left
m_open set, leaving the object in an inconsistent state where code
could treat a closed device as still valid.
This was exposed when reloading a running PlutoSDR device, where GUI
updates could access the partially torn-down device state.
Hopefully fixes#2833 (I don't have a windows machine to test).
Signed-off-by: Robin Getz <rgetz503@gmail.com>