The modem network device is set up by ipa_modem_start().  But its
TX queue is not actually started and endpoints enabled until it is
opened.

So avoid stopping the modem network device TX queue and disabling
endpoints on suspend or stop unless the netdev is marked UP.  And
skip attempting to resume unless it is UP.
Signed-off-by: Alex Elder <elder@linaro.org>
Signed-off-by: David S. Miller <davem@davemloft.net>