igb: count Rx FIFO errors correctly

Don't aggregate rx_no_buffer_count into rx_fifo_errors. RNBC counts
packets that get queued temporarily in the adapter's FIFO. These
packets are not dropped and are not errors. The correct counter
is rx_missed_errors (MPC).

Signed-off-by: Mitch Williams <mitch.a.williams@intel.com>
Signed-off-by: Jeff Kirsher <jeffrey.t.kirsher@intel.com>
Signed-off-by: David S. Miller <davem@davemloft.net>