usb: rndis: Fix zero length skbs in up-link

Commit 7f21ac8 "usb: Add support for rndis uplink aggregation"
introduced bug which can result in zero length skbs being queued
to network stack. Fix bug by checking skb length with data length
instead of message length to determine final skb.

Change-Id: I8bdc3accfbc22527d08382cc1003d726a73bdd9b
Signed-off-by: Vamsi Krishna <vskrishn@codeaurora.org>