staging: hv: Fix the WARN_ON condition in free_net_device()

In a previous commit, 7a09876d, ASSERT was changed to WARN_ON, but
the condition wasn't updated. This patch fixed this error.

Signed-off-by: Haiyang Zhang <haiyangz@microsoft.com>
Signed-off-by: K. Y. Srinivasan <kys@microsoft.com>
Signed-off-by: Hank Janssen <hjanssen@microsoft.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@suse.de>