Bluetooth: Mesh: Fix usage of Identity and NRPA adv options for proxy

Both options are kind of mutually exclusive and can't be used together.
Also, USE_IDENTITY is used for debug purpose and could be useful when
debugging Private Proxy as well. Since CONFIG_BT_MESH_DEBUG_USE_ID_ADDR
is not allowed to be used in production, it is fine to use it for
Private Proxy advertisements as well.

Signed-off-by: Pavel Vasilyev <pavel.vasilyev@nordicsemi.no>