pch_gbe: don't re-set RX_FIFO_ERR flag in napi_poll

If we were in RX_FIFO_ERR state and entered pch_gbe_napi_poll(), we'll
anyway clean some rx space and thus can continue to receive more packets.
Currently, we re-set the RX_FIFO_ERR in situations when we've exhausted our
budget for RX cleaning or cleaned some TX packets. Removing it gives us
+20%-40% speed increase and a lot less of RX_FIFO_ERRors reported.

Signed-off-by: Veaceslav Falico <vfalico@redhat.com>
Signed-off-by: David S. Miller <davem@davemloft.net>