sfc: only use vadaptor stats if firmware is capable

Some of the stats handling code differs based on SR-IOV support,
and SRIOV support is only available if full-featured firmware is
used.
Do not use vadaptor stats if firmware mode is not set to
full-featured.

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