net: rmnet_usb: initialize rmnet before registering with usb

registering with usb before initializing rmnet can trigger
interface probe before rmnet is initialized and can lead to crash.

Change-Id: I51a0b22a210a5878ff21910cdf436b5231f16308
Signed-off-by: Vamsi Krishna <vskrishn@codeaurora.org>
Signed-off-by: Pavankumar Kondeti <pkondeti@codeaurora.org>