usb: gadget: make g_printer enumerate again

usb: gadget: make g_printer enumerate again

commit 5a175bb84d7344fbe5e26cf61b597129e7c80564 upstream.

This was broken in 2e87edf49 ("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.

Acked-by: Michal Nazarewicz <mina86@mina86.com>
Signed-off-by: Sebastian Andrzej Siewior <bigeasy@linutronix.de>
Signed-off-by: Felipe Balbi <balbi@ti.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>