net/sfc: poll MAC stats if periodic DMA is not supported

If periodic DMA statistics feature is absent (particularly,
while running over VF), the PMD must provide an ability to
cope with it using explicit update requests which are kept
restrained according to 'stats_update_period_ms' parameter

Signed-off-by: Ivan Malov <ivan.malov@oktetlabs.ru>
Signed-off-by: Andrew Rybchenko <arybchenko@solarflare.com>
Reviewed-by: Andy Moreton <amoreton@solarflare.com>