Bluetooth: CSIS: Use bt_csis for CSIS client
Bluetooth: CSIS: Use bt_csis for CSIS client

Use the bt_csis struct instead of the
bt_csis_client_svc_inst struct for the CSIS client.
This makes it more similar to not only the CSIS
implementation, but also the other LE Audio
client implementations.

Furthermore, this change will make it easier to use
bt_csis in the API in the future.

Signed-off-by: Emil Gydesen <a20314356615223738233ab4c4eb1cd8cd4146fa@nordicsemi.no>

finikorg/zephyr,finikorg/zephyr,zephyrproject-rtos/zephyr,zephyrproject-rtos/zephyr,zephyrproject-rtos/zephyr,galak/zephyr,finikorg/zephyr,zephyrproject-rtos/zephyr,galak/zephyr,zephyrproject-rtos/zephyr,galak/zephyr,finikorg/zephyr,galak/zephyr,galak/zephyr,finikorg/zephyr