dwc3: gadget: Fix incorrect handling of zlp when sg is enabled

Driver inserts zlp (zero length packet) in the middle of chained
trbs resulting in invalid packet. This is observed during rndis
data transfers when scatter and gather is enabled. It results
in invalid ip packet followed by stall.

Change-Id: I5731b5952f683c069772b4552a53d8c388521ff7
Signed-off-by: Vamsi Krishna <vskrishn@codeaurora.org>