xtensa: iss: extract and constify network callbacks

Instead of storing pointers to callback functions in the
struct iss_net_private::tp move them to struct struct iss_net_ops and
store a const pointer to it. Make static const tuntap_ops structure with
tuntap callbacks and initialize tp.net_ops with it in the tuntap_probe.

Signed-off-by: Max Filippov <jcmvbkbc@gmail.com>