usb: gadget: Remove assigning the function descriptors

The usb_assign_descriptors copies the descriptors depending
on the maximum speed supported by the gadget.
If the function drivers unconditionally assign the
descriptors and the speed is limited to full, the switching
to the same function would lead to a use after free.
Fix this by removing unconditional assigning of descriptors
in the function drivers.

Change-Id: Id283c87d6f3ccf708507afb66a3b93121022e990
Signed-off-by: Sriharsha Allenki <sallenki@codeaurora.org>