Bluetooth: audio: tbs_client: Remove redundant checks
Bluetooth: audio: tbs_client: Remove redundant checks

The CONTAINER_OF does not return NULL, thus the `inst == NULL` checks
are not needed.

Signed-off-by: Mariusz Skamra <c73850bd0bf85e95dce44c2ce1418ca55c1b6c5d@codecoup.pl>

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