USB: gadget: rndis: Add module parameter for DL max packets per xfer

Currently DL aggregation is supported in RNDIS driver and is set to
3 by default. And there is no support to change downlink maximum
packets per transfer at runtime through module parameter. Hence add
module parameter for DL maximum packets per transfer to change it at
runtime.

echo 6 > /sys/module/g_android/parameters/rndis_dl_max_pkt_per_xfer

To disable DL aggregation during runtime,

echo 1 > /sys/module/g_android/parameters/rndis_dl_max_pkt_per_xfer

Change-Id: I06c6226f64c89a49e3e461b54ccdf79c67c0517a
Signed-off-by: Vijayavardhan Vennapusa <vvreddy@codeaurora.org>