f_gsi: Increase number of TRBs and its buffer size

This change increases number of TRBs and its buffer size as below
for better tethering throughput:
RMNET: number of TRBs as 50, and buffer size to 31KB
RNDIS: buffer size to 16KB

Change-Id: I426958e0869e6a9f056ad8a21df56c9e801811d1
Signed-off-by: Mayank Rana <mrana@codeaurora.org>