Bluetooth: Add mgmt_send_event() helper to send to any HCI channel

Currently the mgmt_event() function is only capable of sending to
HCI_CHANNEL_CONTROL. To void having to change all users of it, add a new
mgmt_send_event() function that takes a channel parameter, and make the
old mgmt_event() a wrapper that passes MGMT_CHANNEL_CONTROL to it.

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