hv_netvsc: Fix ethtool change hash key error

Hyper-V hosts require us to disable RSS before changing RSS key,
otherwise the changing request will fail. This patch fixes the
coding error.

Fixes: ff4a4419

 ("netvsc: allow get/set of RSS indirection table")
Reported-by: Wei Hu <weh@microsoft.com>
Signed-off-by: Haiyang Zhang <haiyangz@microsoft.com>
Reviewed-by: Michael Kelley <mikelley@microsoft.com>
[sl: fix up subject line]
Signed-off-by: Sasha Levin <sashal@kernel.org>