net: usb: rmnet_usb_ctrl: Fix NULL pointer dereference

Driver creates device node first and then allocates usb
specific device context. This results into NULL pointer
dereference if userspace entity calls open on device node
and open call accesses usb specific device context which
is not allocated yet. Fix the issue by allocating usb
specific device context before creating device node.

CRs-Fixed: 619963
Change-Id: I2bd254aaf22b00766be72b39c5a55a5aec383588
Signed-off-by: Hemant Kumar <hemantk@codeaurora.org>