sfc: DMA the VF stats only when requested

Firmware does not support a periodic DMA of vadaptor-stats
on VFs, so only update the stats buffer when stats are
requested (when running "ethtool -S" or an ip/ifconfig
command that reports stats).

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