Bluetooth: Provide hci_send_to_flagged_channel helper function

The hci_send_to_flagged_channel helper function can be used to send
packets to all channels that have a certain HCI socket flag set.

This is especially useful for managment events that are limited to
sockets that have first enabled certain functionality. This allows
for filtering of events without confusing existing users.

Signed-off-by: Marcel Holtmann <marcel@holtmann.org>
Signed-off-by: Johan Hedberg <johan.hedberg@intel.com>