net: qmi_wwan: rearranging to prepare for code sharing

Most of the subdriver registration code can be reused for devices
with separate control and data interfaces.  Move the code a bit
around to prepare for such reuse.

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