net: bcmgenet: collect Rx discarded packet count

Bits 31:16 of RDMA_PROD_INDEX contain Rx discarded packet count, which
are the Rx packets that had to be dropped by MAC hardware since there
was no room on the Rx queue. Add code to collect this information into
the netdev stats.

Signed-off-by: Petri Gynther <pgynther@google.com>
Signed-off-by: David S. Miller <davem@davemloft.net>