drivers/net/hyperv/netvsc.c: remove unnecessary null test before kfree

Fix checkpatch warning:
WARNING: kfree(NULL) is safe this check is probably not required

Cc: Haiyang Zhang <haiyangz@microsoft.com>
Cc: netdev@vger.kernel.org
Signed-off-by: Fabian Frederick <fabf@skynet.be>
Signed-off-by: David S. Miller <davem@davemloft.net>