Bluetooth: Move mgmt response convenience functions to a better location

The settings_rsp and cmd_status_rsp functions can be useful for all mgmt
command handlers when asynchronous request callbacks are used. They will
e.g. be used by subsequent patches to change set_le to use an async
request as well as a new set_advertising command. Therefore, move them
higher up in the mgmt.c file to avoid unnecessary forward declarations
or mixing this trivial change with other patches.

Signed-off-by: Johan Hedberg <johan.hedberg@intel.com>
Acked-by: Marcel Holtmann <marcel@holtmann.org>
Signed-off-by: Gustavo Padovan <gustavo.padovan@collabora.co.uk>