IB/hfi1: Use free_netdev() in hfi1_netdev_free()

IB/hfi1: Use free_netdev() in hfi1_netdev_free()

dummy_netdev shold be freed by free_netdev() instead of kfree(). Also
remove unneeded variable 'priv'

Fixes: 4730f4a6c6b2 ("IB/hfi1: Activate the dummy netdev")
Link: https://lore.kernel.org/r/20200602061635.31224-1-yuehaibing@huawei.com
Signed-off-by: YueHaibing <yuehaibing@huawei.com>
Reported-by: kbuild test robot <lkp@intel.com>
Reported-by: Dan Carpenter <dan.carpenter@oracle.com>
Reviewed-by: Dennis Dalessandro <dennis.dalessandro@intel.com>
Signed-off-by: Jason Gunthorpe <jgg@mellanox.com>