Bluetooth: ISO: ensure no dangling hcon references in iso_conn

Bluetooth: ISO: ensure no dangling hcon references in iso_conn

After iso_conn_del(), ISO sockets should not dereference the hcon any
more.  Currently, clearing iso_conn::hcon relies on iso_conn_del()
releasing the last reference to the iso_conn.

Simplify this by explicitly clearing conn->hcon in iso_conn_del(), to
avoid more complex reasoning on races about who holds the last
reference.

Signed-off-by: Pauli Virtanen <pav@iki.fi>
Signed-off-by: Luiz Augusto von Dentz <luiz.von.dentz@intel.com>