RDMA/nes: Don't generate async events for unregistered devices

nes_port_ibevent() should not be called when the nes RDMA device is not
registered with the RDMA core.  Add missing checks of of_device_registered flag.

Signed-off-by: Maciej Sosnowski <maciej.sosnowski@intel.com>
Signed-off-by: Roland Dreier <roland@purestorage.com>