drivers/nble: Make sure gatt_private is reset properly

gatt_private should be reseted whenever used otherwise other commands
may fail when attempt to use.

Change-Id: I8bdda3d5136368a8f37fd9e2afd97e0957c4e04e
Signed-off-by: Luiz Augusto von Dentz <luiz.von.dentz@intel.com>