usb: gadget: f_gsi: Support IPA scatter-gather for DL

Add support for IPA scatter-gather functionality by using
2K DL buffers. Aggregation size for RNDIS is 9k and 16k
for MBIM,ECM,RMNET. Also add a module param to change
the aggregation size before usb plugin.
Module param can be updated as

echo 8192 > /sys/module/g_android/parameters/gsi_in_aggr_size

Change-Id: I0ea73304080b14398bbf509e1454ef22adbab933
Signed-off-by: Azhar Shaikh <azhars@codeaurora.org>