Bluetooth: host: Fix notify multiple issue

When multiple notify feature is enabled, notifications are pushed
twice. Host sends multiple notify with BT_ATT_OP_NOTIFY_MULT and
then continues to send regular notification with BT_ATT_OP_NOTIFY.

Fixes: https://github.com/zephyrproject-rtos/zephyr/issues/40546



Signed-off-by: Ilhan Ates <ilhan.ates@nordicsemi.no>