net: qmi_wwan: call subdriver with control intf only

This fixes a hang on suspend due to calling wdm_suspend on
the unregistered data interface. The hang should have been
a NULL pointer reference had it not been for a logic error
in the cdc_wdm code.

  commit 230718bd net: qmi_wwan: bind to both control and data interface

changed qmi_wwan to use cdc_wdm as a subdriver for devices with
a two-interface QMI/wwan function.  The commit failed to update
qmi_wwan_suspend and qmi_wwan_resume, which were written to handle
either a single combined interface function, or no subdriver at all.

The result was that we called into the subdriver both when the
control interface was suspended and when the data interface was
suspended.  Calling the subdriver suspend function with an
unregistered interface is not supported and will make the
subdriver bug out.

Signed-off-by: Bjørn Mork <bjorn@mork.no>
Signed-off-by: David S. Miller <davem@davemloft.net>