ionic: prevent early watchdog check

In one corner case scenario, the driver device lif setup can
get delayed such that the ionic_watchdog_cb() timer goes off
before the ionic->lif is set, thus causing a NULL pointer panic.
We catch the problem by checking for a NULL lif just a little
earlier in the callback.

Signed-off-by: Shannon Nelson <snelson@pensando.io>
Signed-off-by: David S. Miller <davem@davemloft.net>