net: wwan: mhi: fix memory leak in mhi_mbim_dellink

In the Linux kernel, the following vulnerability has been resolved: net: wwan: mhi: fix memory leak in mhi_mbim_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 sets needs_free_netdev to true when registers network device, which makes netdev subsystem call free_netdev() automatically after unregister_netdevice().