bnxt_en: Accumulate all counters.

Now that we have the infrastructure in place, add the new function
bnxt_accumulate_all_stats() to periodically accumulate and check for
counter rollover of all ring stats and port stats.

A chip bug was also discovered that could cause some ring counters to
become 0 during DMA.  Workaround by ignoring zeros on the affected
chips.

Some older frimware will reset port counters during ifdown.  We need
to check for that and free the accumulated port counters during ifdown
to prevent bogus counter overflow detection during ifup.

Reviewed-by: Vasundhara Volam <vasundhara-v.volam@broadcom.com>
Signed-off-by: Michael Chan <michael.chan@broadcom.com>
Signed-off-by: David S. Miller <davem@davemloft.net>