usb: gadget: rndis: Limit DL aggregation to 5 packets

Observing data stalls in down link direction when 7 or more small
(size of 56 Bytes, for example tcp acks) ip packets. To avoid data
stalls, limit downlink aggregation to 5 for rndis

Change-Id: I3bdd932ec4fc37df965f44b29669f7137c1f7f83
Signed-off-by: Vamsi Krishna <vskrishn@codeaurora.org>