Bluetooth: Audio: Update VCS Client conn check return value
Bluetooth: Audio: Update VCS Client conn check return value

Instead of return -ENOTCONN they will now return -EINVAL as
that is a more appropriate return value for checking if
the pointer is NULL.

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

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