Bluetooth: Convert discovery commands to use cmd_complete callback

This patch converts the Start/Stop Discovery mgmt commands to use the
cmd_complete callback of struct pending_cmd. Since both of these
commands return the same parameters as they take as input we can use the
existing generic_cmd_complete() helper for this.

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