usb: gadget: Register RMNET SMD control channel only on protocol connect

The RMNET USB function driver uses SMD channel for transmitting control
messages to the modem processor. Move the registration of the platform
driver which handle the SMD channel to the stage in which the RMNET
function driver is actually got activated by the host which selected
the configuration which contain the RMNET function driver.

This fix is required to prevent cases in which other protocols, as
MBIM for example, which exists in other USB configuration, may need
the same SMD channel which is occupied by the RMNET, since the RMNET
grab the channel on initialization of the function driver rather than
upon activating it.

CRs-Fixed: 435979
Change-Id: I40667c83ca6ff6402a8d6f69a62f1dc42bd265d0
Signed-off-by: Ido Shayevitz <idos@codeaurora.org>
Signed-off-by: Tarun Gupta <tarung@codeaurora.org>