Bluetooth: hci_conn: Refactor hci_bind_bis() since it always succeeds

The compiler thinks "conn" might be NULL after a call to hci_bind_bis(),
which cannot happen. Avoid any confusion by just making it not return a
value since it cannot fail. Fixes the warnings seen with GCC 13:

In function 'arch_atomic_dec_and_test',
    inlined from 'atomic_dec_and_test' at ../include/linux/atomic/atomic-instrumented.h:576:9,
    inlined from 'hci_conn_drop' at ../include/net/bluetooth/hci_core.h:1391:6,
    inlined from 'hci_connect_bis' at ../net/bluetooth/hci_conn.c:2124:3:
../arch/x86/include/asm/rmwcc.h:37:9: warning: array subscript 0 is outside array bounds of 'atomic_t[0]' [-Warray-bounds=]
   37 |         asm volatile (fullop CC_SET(cc) \
      |         ^~~
...
In function 'hci_connect_bis':
cc1: note: source object is likely at address zero

Fixes: eca0ae4a

 ("Bluetooth: Add initial implementation of BIS connections")
Cc: Luiz Augusto von Dentz <luiz.von.dentz@intel.com>
Cc: Marcel Holtmann <marcel@holtmann.org>
Cc: Johan Hedberg <johan.hedberg@gmail.com>
Cc: "David S. Miller" <davem@davemloft.net>
Cc: Eric Dumazet <edumazet@google.com>
Cc: Jakub Kicinski <kuba@kernel.org>
Cc: Paolo Abeni <pabeni@redhat.com>
Cc: linux-bluetooth@vger.kernel.org
Cc: netdev@vger.kernel.org
Signed-off-by: Kees Cook <keescook@chromium.org>
Reviewed-by: Simon Horman <simon.horman@corigine.com>
Signed-off-by: Luiz Augusto von Dentz <luiz.von.dentz@intel.com>