Bluetooth: drivers/nble: Fix calling cmd from discov callback

Private gatt data from connection is indicator if another operation is
pending. Some att commands may be called within discovery callback
like subscription on ccc (within characteristic discovery procedure).
Without clearing conn private gatt data member, leads to rejection of
called in callback att command due to being "busy".

Change-Id: I72a35ed82dac2f0538f2d1637163bd2302f0b072
Signed-off-by: Grzegorz Kolodziejczyk <grzegorz.kolodziejczyk@tieto.com>