sfc: nicer log message on Siena SR-IOV probe fail

We expect that MC_CMD_SRIOV will fail if the card has no VFs configured.
So output a readable message instead of a cryptic MCDI error.

Signed-off-by: Edward Cree <ecree@solarflare.com>
Signed-off-by: David S. Miller <davem@davemloft.net>