enic: unmask intr only when napi is complete

In case of busy poll, napi_complete_done returns false and does not
dequeue napi. In this case do not unmask the intr. We are guaranteed
napi is called again. This reduces unnecessary iowrites.

Signed-off-by: Govindarajulu Varadarajan <gvaradar@cisco.com>
Signed-off-by: David S. Miller <davem@davemloft.net>