usb: gadget: uvc: Generalise helper functions for reuse

The __uvcg_*frm_intrv() helper functions can be helpful when adding
support for similar attributes. Generalise the functions and
move them higher in the file for better coverage.

Signed-off-by: Daniel Scally <dan.scally@ideasonboard.com>
Link: https://lore.kernel.org/r/20230206161802.892954-3-dan.scally@ideasonboard.com
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>