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

'Commit 72ffa18b ("usb: Add support for rndis uplink
aggregation")' introduced issue which can result in zero length
skbs being queued to network stack. Fix this 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>
Signed-off-by: Ajay Agarwal <ajaya@codeaurora.org>