usb: gadget: OS descriptors configfs cleanup

A number of variables serve a generic purpose of handling
"compatible id" and "subcompatible id", but the names suggest they
are for rndis only. Rename to reflect variables' purpose.

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