xtensa: iss: clean up per-device locking in network driver

Per-device locking in the ISS network driver is used to protect poll
timer and stats updates. Stat collection is not protected.
Remove per-device locking everywhere except the stats updates. Replace
ndo_get_stats callback with ndo_get_stats64 and use proper locking there
as well.
As a side effect this fixes possible deadlock between iss_net_close
and iss_net_timer.

Reported by: Duoming Zhou <duoming@zju.edu.cn>
Signed-off-by: Max Filippov <jcmvbkbc@gmail.com>