usb: gadget: uvc: Allow linking function to string descs

usb: gadget: uvc: Allow linking function to string descs

Currently the string descriptors for the IAD and VideoStreaming
Interfaces are hardcoded into f_uvc. Now that we can create arbitrary
string descriptors, add a mechanism to define string descriptors for
the IAD, VC and VS interfaces by linking to the appropriate directory
at function level.

Signed-off-by: Daniel Scally <dan.scally@ideasonboard.com>
Link: https://lore.kernel.org/r/20230206161802.892954-11-dan.scally@ideasonboard.com
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>