net: macb: Re-enable RX interrupt only when RX is done

When data is received during the driver processing received data the
NAPI is re-scheduled. In that case the RX interrupt should not be
re-enabled.

Signed-off-by: Soren Brinkmann <soren.brinkmann@xilinx.com>
Signed-off-by: David S. Miller <davem@davemloft.net>