ovs: fix dp check in ovs_dp_reset_user_features

This fixes crash when userspace does "ovs-dpctl add-dp dev" where dev is
existing non-dp netdevice.

Introduced by:
commit 44da5ae5


"openvswitch: Drop user features if old user space attempted to create datapath"

Signed-off-by: Jiri Pirko <jiri@resnulli.us>
Signed-off-by: Jesse Gross <jesse@nicira.com>