Bluetooth: Add callback to create proper cmd_complete events

We've got a couple of generic scenarios where all pending mgmt commands
are processed and responses are sent to them. These scenarios are
powering off the adapter and removing the adapter. So far the code has
been generating cmd_status responses with NOT_POWERED and INVALID_INDEX
resposes respectively, but this violates the mgmt specification for
commands that should always generate a cmd_complete.

This patch adds support for specifying a callback for the pending_cmd
context that each command handler can use for command-specific
cmd_complete event generation. The actual per-command event generators
will come in subsequent patches.

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