Add support for netvsc build without CONFIG_SYSFS flag

This change ensures the driver can be built successfully without the
CONFIG_SYSFS flag.
MS-TFS: 182270

Signed-off-by: Haiyang Zhang <haiyangz@microsoft.com>
Reviewed-by: K. Y. Srinivasan <kys@microsoft.com>
Signed-off-by: David S. Miller <davem@davemloft.net>