drivers/nble: Fix GATT Server Indications

This fixes GATT Server indications.
conn handle shall be set to the valid handle, not 0xffff.
If conn passed to bt_gatt_indicate is NULL, indication will be sent
to every client subscribed.

Fix related to ZEP-403

Change-Id: Ie36b957fddfbc5485318bba649d15349e9293868
Signed-off-by: Mariusz Skamra <mariusz.skamra@tieto.com>