openvswitch: Fix return value check in ovs_meter_cmd_features()

In case of error, the function ovs_meter_cmd_reply_start() returns
ERR_PTR() not NULL. The NULL test in the return value check should
be replaced with IS_ERR().

Fixes: 96fbc13d ("openvswitch: Add meter infrastructure")
Signed-off-by: Wei Yongjun <weiyongjun1@huawei.com>
Acked-by: Pravin B Shelar <pshelar@ovn.org>
Signed-off-by: David S. Miller <davem@davemloft.net>