net: rmnet_usb: silence URB submission errors during disconnect

URB submissions will fail with -ENODEV while the device is being
disconnected.  The current code prints error message for an URB
submission failure.  If too many URBs are submitted during the
disconnect window, then target is crashing due to excessive
console logging.

CRs-Fixed: 507556
Change-Id: I287c5021bacb2238b62bf78a8ddf887f588b6f16
Signed-off-by: Pavankumar Kondeti <pkondeti@codeaurora.org>