Internal statistics system on chip never gets reset until hardware
reboot. This is quite inconvenient in terms of ethtool statistics usage.

This patch implements incremental statistics update inside of
service callback.

Upon nic initialization, first request is done to fetch
initial stat data, current collected stat data gets cleared.
Internal statistics mailbox readout is improved to save space and
increase readability
Signed-off-by: Pavel Belous <pavel.belous@aquantia.com>
Signed-off-by: Igor Russkikh <igor.russkikh@aquantia.com>
Signed-off-by: David S. Miller <davem@davemloft.net>