sfc: update netdevice statistics to use vadaptor stats

The netdevice statistics (in /proc/net/dev) are per-function
stats so they must use the vadaptor stats. Change the use of
MAC stats to vadaptor stats, and remove any statistics that
can only be measured per-port.  All stats that are removed
will be shown as zeroes when these statistics are displayed.

Signed-off-by: Shradha Shah <sshah@solarflare.com>
Signed-off-by: David S. Miller <davem@davemloft.net>