modules: nrf_wifi: Fix interface down hang

When QSPI LPM is enabled, QSPI will already be uninitialized, and during
deinit if uninitialized is being called again then it leads to a hang as
it waits for mem busy check to pass which it won't.

Fix by checking if the device is initialized before calling
uninitialize.

Signed-off-by: Chaitanya Tata <Chaitanya.Tata@nordicsemi.no>