Bluetooth: ISO: Fix parent socket leak in iso_conn_ready()

iso_get_sock() returns the parent socket with a reference held, which is
dropped by sock_put() once the child socket has been set up. The error
path taken when iso_sock_alloc() fails only calls release_sock() and
returns, leaking the reference and thus the parent socket itself.

Drop the reference on that path as well.

Fixes: fa224d0c094a ("Bluetooth: ISO: Reassociate a socket with an active BIS")
Signed-off-by: Luiz Augusto von Dentz <luiz.von.dentz@intel.com>