Bluetooth: Convert Disconnect mgmt command to use cmd_complete callback

This patch converts the Disconnect mgmt command to take advantage of the
new cmd_complete callback that's part of the pending_cmd struct. There
are many commands whose response parameters map 1:1 to the command
parameters and Disconnect is one of them. This patch adds a
generic_cmd_complete() function for such commands that can be reused in
subsequent patches.

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