Bluetooth: Mesh: Fix cfg_cli KRP not working for key_net_idx != 0

Initializes the `net_idx` field of the ack context properly for KRP
functions in the Config Client.

This fixes a problem where `bt_mesh_cfg_cli_krp_get` and
`bt_mesh_cfg_cli_krp_set` would always fail the ack ctx comparison if
`key_net_idx` was != 0.

Signed-off-by: Ludvig Jordet <ludvig.jordet@nordicsemi.no>