net: bcmgenet: fix accounting of packet drops vs errors

bcmgenet driver needs to separate packet drops from packet errors.

When the driver has to drop a *good* packet, due to lack of buffers or
replacement skbs, increment only dev->stats.[rx|tx]_dropped.

When the driver encounters a bad Rx packet or Tx error, increment only
dev->stats.[rx|tx]_errors + relevant detailed error counter.

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