usb: gadget: make usb functions to load before gadget driver

The current ordering in makefile makes gadget
drivers be loaded before usb functions which
causes usb_get_function_instance() to fail when
gadget modules are statically linked to the
kernel binary.

Changed the ordering here so that USB functions
are loaded before gadget drivers.

Note that this is only a temporary solution and
a more robust fix is needed in the long run.

Signed-off-by: Kishon Vijay Abraham I <kishon@ti.com>
Signed-off-by: Felipe Balbi <balbi@ti.com>