usb: gadget: uvc: configfs: Simplify attributes macros

Several macros used to define attributes and their access functions take
multiple arguments to specify endianness and string conversion
functions, based on the size of the attribute. This can be simplified by
passing the number of bits explicitly, and constructing the name of the
functions internally.

The UVCG_OPTS_ATTR macro can be simplified further as all fields it
deals with are unsigned int.

Signed-off-by: Laurent Pinchart <laurent.pinchart@ideasonboard.com>
Reviewed-by: Paul Elder <paul.elder@ideasonboard.com>
Tested-by: Paul Elder <paul.elder@ideasonboard.com>