8139cp: fix missing napi_gro_flush.

The driver uses __napi_complete and napi_gro_receive. Without it, the
driver hits the BUG_ON(n->gro_list) assertion hard in __napi_complete.

Signed-off-by: Francois Romieu <romieu@fr.zoreil.com>
Tested-by: Marin Glibic <zhilla2@gmail.com>
Signed-off-by: David S. Miller <davem@davemloft.net>