usb: gadget: make g_printer use composite

This patch converts the g_printer to make use of the compoiste framework
for descriptor parsing instead of its own implementation of it.
This gadget contains now one function which is the printer gadget.
Compile tested only.
Signed-off-by: Sebastian Andrzej Siewior <sebastian@breakpoint.cc>
Signed-off-by: Felipe Balbi <balbi@ti.com>