Bluetooth: Classic: HFP_AG: Improve the SCO management

There are two issues found,
Issue 1, if the SCO connection has been established, the SCO connected
event will not be notified. Then the call that waiting for the SCO
connected event will not be updated. And the call will be rejected due
to the timeout.
Issue 2, the SCO con object will be cleared when SLC is disconnecting
and the SCO connection has been established. It causes the SCO connect
object will be `NULL` in the SCO disconnected callback.

Add a new field `sco_conn` to keep the SCO connect object. Manage the
SCO connection via the field `ag->sco_conn`.

When creating the SCO connection, keep the created SCO connect object
to `ag->sco_conn`. And if the `ag->sco_chan.sco` is `NULL`, it means
the SCO connection has been established. Just update the call that
waiting for the SCO connected event directly.
Increase the `ref` of SCO connect object and pass to the field
`ag->sco_conn` if the `ag->sco_conn` is `NULL`.
Decrease the `ref` of SCO connect object if the `ag->sco_conn` is not
`NULL` when disconnect the SCO connection or process SCO disconnected
event.

Signed-off-by: Lyle Zhu <lyle.zhu@nxp.com>