Bluetooth: host: Fix `gatt_notify_mult` by using a delayed work item

The intended behavior is that calling `gatt_notify_mult` in a loop will
result in a `BT_ATT_OP_NOTIFY_MULT` containing all the handles and their
data.

What was happening instead is that the caller thread was a lower priority
than the system workqueue, and thus multiple `BT_ATT_OP_NOTIFY_MULT` PDUs
were being sent over the air instead of one.

Submitting the work item with a delay side-steps this particular issue.

Signed-off-by: Jonathan Rico <jonathan.rico@nordicsemi.no>