usb: f_gsi: Use required RNDIS IAD descriptors with full speed descriptors

gsi_eth_fs_function is array of pointers to set of RNDIS interface
related descriptors. gsi_eth_fs_function's first descriptor is
pointing itself instead of required RNDIS IAD descriptor. This
results into out-of-bound read access while copying these set of
descriptors with usb_copy_descriptors() API. Fix this issue by
using required RNDIS IAD descriptor here.

Change-Id: Ic604221febc43eb8a22d8de99fb8cead74e13b41
Signed-off-by: Mayank Rana <mrana@codeaurora.org>