usb: f_gsi: Increase max control transfer size to 8K

Max packet size that GNSS can use is 8K.
Increase the max buff size support in driver from 4K
to 8K so that transfers larger than 4K are not failed.

Change-Id: If74a7d9994101a55d27ff92258b8ab7ef2d54a50
Signed-off-by: Manu Gautam <mgautam@codeaurora.org>