usb: gadget: OS descriptors: provide interface directory names

Function's interface directories need to be created when the function
directory is created, but interface numbers are not known until
the gadget is ready and bound to udc, so we cannot use numbers
as part of interface directory names.
Let the client decide what names to use.

Signed-off-by: Andrzej Pietrasiewicz <andrzej.p@samsung.com>
Signed-off-by: Felipe Balbi <balbi@ti.com>