[NIU]: Fix slowpath interrupt handling.

niu_slowpath_interrupt() expects values to be setup in lp->{v0,v1,v2}
but they aren't.  That's only done by niu_schedule_napi() which is
done later in the interrupt path.

If niu_rx_error() returns zero, and v0 is clear, hit the
RX_DMA_CTL_STATE register with a RX_DMA_CTL_STAT_MEX.

Only emit verbose RX error logs if a fatal channel or port error is
signalled.  Other cases will be recorded into statistics by
niu_log_rxchan_errors().

Signed-off-by: David S. Miller <davem@davemloft.net>