Bluetooth: Audio: Fix ASCS CP subscribe in discovery

During discovery the unicast client subscribes to the
ASCS CP characteristic. However, this was done asynchronously
at the end of the discovery, and the discovery complete
callback was called before the subscription was done.

Since the unicast client relies on the auto CCC discovery,
the actual subscription takes place a few connection intervals
after the unicast_client_ep_set_cp function was called. The
result of this is that the actual subscription was missing
when the unicast client application starts performing
BAP operations.

The change done here postpones the discovery callback
until after the CP subscription has taken place, ensuring
that the unicast client correctly receives the CP notifications
when performing BAP operations. This also allows the unicast
client to use the discovery parameter in the
bt_audio_discover_params struct, rather than yet another
discovery parameter struct, saving some memory.

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