Bluetooth: Audio: Add common bt_audio_get_max_ntf_size

Add a common function to get the maximum notification
size, instead of having multiple implementations of this.
Ideally this function should be moved to the GATT API,
but for now just implement it for LE audio and then move to
GATT later.

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