Bluetooth: Audio: Update VOCS conn check return value
Bluetooth: Audio: Update VOCS conn check return value

Change the return value from -ENOTCONN to -EINVAL as that
is a more appropriate return value for checking a NULL
pointer.

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

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