net: mhi: Fix memory leak in mhi_net_dellink()

MHI driver registers network device without setting the
needs_free_netdev flag, and does NOT call free_netdev() when
unregisters network device, which causes a memory leak.

This patch calls free_netdev() to fix it since netdev_priv
is used after unregister.

Fixes: 13adac03

 ("net: mhi_net: Register wwan_ops for link creation")
Signed-off-by: Wei Yongjun <weiyongjun1@huawei.com>
Signed-off-by: David S. Miller <davem@davemloft.net>