usb: dwc3: gadget: Fix endpoint init for reserving gsi endpoint

gsi endpoint is reserved using endpoint address instead of
endpoint number. This would name other endpoints as gsi endpoint
as well.

Change-Id: I15f86b96b1a9a69e895fac5a2fe4f90fe6bb8a81
Signed-off-by: Hemant Kumar <hemantk@codeaurora.org>