usb: gadget: f_uac2: allow changing interface name via configfs

usb: gadget: f_uac2: allow changing interface name via configfs

This adds "function_name" configfs entry to change string value
of the iInterface field. This field will be shown in Windows' audio
settings panel, so being able to change it is useful. It will default
to "Source/Sink" just as before.

Signed-off-by: Yunhao Tian <[email protected]>
Link: https://lore.kernel.org/r/[email protected]
Signed-off-by: Greg Kroah-Hartman <[email protected]>