sfc: Enable VF's via a write to the sysfs file sriov_numvfs

This patch adds support for the use of sriov_configure on EF10
to enable Virtual Functions while the driver is loaded.

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