Assign the ipa->modem_netdev and endpoint->netdev pointers *before*
registering the network device.  As soon as the device is
registered it can be opened, and by that time we'll want those
pointers valid.

Similarly, don't make those pointers NULL until *after* the modem
network device is unregistered in ipa_modem_stop().
Signed-off-by: Alex Elder <elder@linaro.org>
Signed-off-by: David S. Miller <davem@davemloft.net>