usb: gadget: gsi: Increase Rmnet UL buffer size from 16K to 31K

IPA Rmnet interface supports packet aggregation/de-aggregation up to 31K.
Hence, increase buffer size given to IPA from 16K to 31K.

Change-Id: Ib136bee28b324f5b665a6309228b823e37079862
Signed-off-by: Vamsi Krishna Samavedam <vskrishn@codeaurora.org>