net: mhi: Fix memory leak in mhi_net_dellink()
In the Linux kernel, the following vulnerability has been resolved:

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.