sh_eth: add NAPI support

The driver hasn't used NAPI so far; implement its support at last...

The patch was tested on Renesas R8A77781 BOCK-W board.

Signed-off-by: Sergei Shtylyov <sergei.shtylyov@cogentembedded.com>
Signed-off-by: David S. Miller <davem@davemloft.net>