sfc: error code if SRIOV cannot be disabled

If SRIOV cannot be disabled during device removal or module unloading,
return error code so it can be logged properly in the calling function.

Note that this can only happen if any VF is currently attached to a
guest using Xen, but not with vfio/KVM. Despite that in that case the
VFs won't work properly with PF removed and/or the module unloaded, I
have let it as is because I don't know what side effects may have
changing it, and also it seems to be the same that other drivers are
doing in this situation.

In the case of being called during SRIOV reconfiguration, the behavior
hasn't changed because the function is called with force=false.

Signed-off-by: Íñigo Huguet <ihuguet@redhat.com>
Signed-off-by: David S. Miller <davem@davemloft.net>