net: bcmgenet: log RX buffer allocation and RX/TX dma failures

To help troubleshoot heavy memory pressure conditions, add a bunch of
statistics counter to log RX buffer allocation and RX/TX DMA mapping
failures. These are reported like any other counters through the ethtool
stats interface.

Signed-off-by: Florian Fainelli <f.fainelli@gmail.com>
Signed-off-by: David S. Miller <davem@davemloft.net>