usb: gadget: make g_printer enumerate again

This was broken in 2e87edf4 ("usb: gadget: make g_printer use
composite").
The USB-strings were not setup properly and were not used. No function
was added which results in an empty USB config.
While fixing this, the interface number is now auto generated and not
hard coded to 0.

Cc: stable@vger.kernel.org # v3.5
Acked-by: Michal Nazarewicz <mina86@mina86.com>
Signed-off-by: Sebastian Andrzej Siewior <bigeasy@linutronix.de>
Signed-off-by: Felipe Balbi <balbi@ti.com>