net: qmi_wwan: define a structure for driver specific state

usbnet allocates a fixed size array for minidriver specific
state.  Naming the fields and taking advantage of type checking
is a bit more failsafe than casting array elements each time
they are referenced.

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