Bluetooth: BAP: Broadcast Source: Modify when enabling state is set

Modify when the BT_BAP_EP_STATE_ENABLING state is set during
bt_bap_broadcast_source_start.

The reason why the state is now set before the call to
bt_iso_big_create, is to handle the case where the BIG
created event actually happens and is handled (by another
thread) faster than the state change is done, which could
cause an invalid state check if the streaming state is
attempted to be set before the enabling state.

Since the events from the controller may be handled by a
different thread, we should not assume that this function returns
before the event is handled.

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