usb: gadget: Update functions for SSP descriptors

This change updates USB function drivers to assign SSP descriptors
using usb_assign_descriptors() API. USB SSP descriptors are needed
to enumerate these functions as different composite device in
super speed plus mode.

Change-Id: Iea4d623afb2bf572358d1cc1d0673f20f2d7d5aa
Signed-off-by: Mayank Rana <mrana@codeaurora.org>