Bluetooth: CAP: Fix uninitialized values in broadcast start

When CONFIG_BT_ISO_TEST_PARAMS is enabled then the
bt_bap_broadcast_source_param in
bt_cap_initiator_broadcast_audio_create had uninitialized values.

A general and future proof solution for this is to simply initialize
the entire struct to 0.

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