Bluetooth: MGMT: mesh_send: check instances prior disabling advertising

The unconditional call of hci_disable_advertising_sync() in
mesh_send_done_sync() also disables other LE advertisings (non mesh
related).

I am not sure whether this call is required at all, but checking the
adv_instances list (like done at other places) seems to solve the
problem.

Fixes: b338d917 ("Bluetooth: Implement support for Mesh")
Cc: stable@vger.kernel.org
Signed-off-by: Christian Eggers <ceggers@arri.de>
Signed-off-by: Luiz Augusto von Dentz <luiz.von.dentz@intel.com>