r8169: use napi_complete_done return value

Consider the return value of napi_complete_done(), this allows users to
use the gro_flush_timeout sysfs attribute as an alternative to classic
interrupt coalescing.

Signed-off-by: Heiner Kallweit <hkallweit1@gmail.com>
Signed-off-by: David S. Miller <davem@davemloft.net>