net: rmnet_usb_ctrl: Fix bug in handling interface disconnect

RMNET control driver sets the device interface pointer to NULL
during disconnect and then unlinks the tx URBs. This causes the
tx completion handler to get called and access the device interface
pointer which is already NULL. Fix the issue by adding a flag
which keeps track of device disconnection and do not set device
interface pointer to NULL.

CRs-Fixed: 373617
Change-Id: I4991c1f437f1ce031c634109aca10e170ca6290b
Signed-off-by: Hemant Kumar <hemantk@codeaurora.org>