USB: gadget: Add ID numbers to gadget names

Putting USB gadgets on a new bus of their own encounters a problem
when multiple gadgets are present: They all have the same name!  The
driver core fails with a "sys: cannot create duplicate filename" error
when creating any of the /sys/bus/gadget/devices/<gadget-name>
symbolic links after the first.

This patch fixes the problem by adding a ".N" suffix to each gadget's
name when the gadget is registered (where N is a unique ID number),
thus making the names distinct.

Reported-and-tested-by: Geert Uytterhoeven <geert+renesas@glider.be>
Signed-off-by: Alan Stern <stern@rowland.harvard.edu>
Reviewed-by: Geert Uytterhoeven <geert+renesas@glider.be>
Fixes: fc274c1e ("USB: gadget: Add a new bus for gadgets")
Link: https://lore.kernel.org/r/YnqKAXKyp9Vq/pbn@rowland.harvard.edu


Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>