bnxt_en: Improve RX buffer error handling.

When hardware reports RX buffer errors, the latest 57500 chips do not
require reset.  The packet is discarded by the hardware and the
ring will continue to operate.

Also, add an rx_buf_errors counter for this type of error.  It can help
the user to identify if the aggregation ring is too small.

Signed-off-by: Michael Chan <michael.chan@broadcom.com>
Signed-off-by: David S. Miller <davem@davemloft.net>