net: lantiq: Use napi_complete_done()

Use napi_complete_done() and activate the interrupts when this function
returns true. This way the generic NAPI code can take care of activating
the interrupts.

Signed-off-by: Hauke Mehrtens <hauke@hauke-m.de>
Signed-off-by: David S. Miller <davem@davemloft.net>