net: usb: Handle Zero length packet

RMNET control driver does not handle zero length packet and
passes it on as QMI command response received from mdm device
to userspace. mdm device is erroneously sending zero length
packet to the RMNET control driver which userspace treats as
an error. Hence add a check for zero length packet and do not
pass it on to userspace.

Change-Id: Id18e2f6ffca1692243a3d89a25bb3ccafd7d5b89
Signed-off-by: Hemant Kumar <hemantk@codeaurora.org>