usb: gadget: f_gsi: Fix alternate index returned by gsi_get_alt

RNDIS, RMNET and DPL only supports alternate index 0. Instead of
returning value of data_interface_up which is used for interfaces
supporting non-zero alternate index, simply return 0.

CRs-Fixed: 2032555
Change-Id: Id6588536f9f7e9221d4d896edec2276f7c11fb8f
Signed-off-by: Devdutt Patnaik <dpatnaik@codeaurora.org>