usb: gadget: configfs: Rename struct gadget_strings

The struct gadget_strings really represents a single language in
configfs. Rename it to make that more clear.

Signed-off-by: Daniel Scally <dan.scally@ideasonboard.com>
Link: https://lore.kernel.org/r/20230206161802.892954-6-dan.scally@ideasonboard.com


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