MTU changes, ring size changes, etc cause the chip to be reset and the
statisctics flushed.  To keep track of the accumulated statistics, we
add code to save the whole statistics block before reset.  We also
modify the macros and statistics functions to return the sum of the
saved and current counters.

Based on original patch by Breno Leitao <leitao@linux.vnet.ibm.com>
Signed-off-by: Michael Chan <mchan@broadcom.com>
Signed-off-by: David S. Miller <davem@davemloft.net>