dwc3: gadget: Increase TXFIFO size as 6KB for GSI IN endpoint

For better tethering throughput, increase TXFIFO size as 6KB for
GSI IN endpoint which enables bursting of more than 2.

Change-Id: Ia6b86b90fb874bdc06065de51e65a9872022b372
Signed-off-by: Mayank Rana <mrana@codeaurora.org>
Signed-off-by: Hemant Kumar <hemantk@codeaurora.org>