drivers: net: mhi: fix error path in mhi_net_newlink

Fix double free_netdev when mhi_prepare_for_transfer fails.

Fixes: 3ffec6a1 ("net: Add mhi-net driver")
Signed-off-by: Daniele Palmas <dnlplm@gmail.com>
Reviewed-by: Manivannan Sadhasivam <mani@kernel.org>
Reviewed-by: Loic Poulain <loic.poulain@linaro.org>
Signed-off-by: David S. Miller <davem@davemloft.net>