be2net: Add a dma_mapping_error counter in ethtool

Add a dma_mapping_error counter to count the number of packets dropped
due to DMA mapping errors.

Signed-off-by: Vasundhara Volam <vasundhara.volam@emulex.com>
Signed-off-by: Sathya Perla <sathya.perla@emulex.com>
Signed-off-by: David S. Miller <davem@davemloft.net>