Bluetooth: BAP: Fix bad use of bt_ascs_ase_status

The bt_ascs_ase_status was stored directly in the
struct bt_bap_ep, which was an issue as
bt_ascs_ase_status is a variable sized struct.
The struct could have been moved to the end of
bt_bap_ep to avoid the issues it caused, but
there is no actual reason to use that struct
rather than storing the ID and state as
seperate values.

The ascs_ep_get_state exists but was not widely
used in ascs. Modify places to use the function
instead of directly accessing the state.

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