usb-gadget/uvc: use per-attribute show and store methods

UVC is a little different from other configfs consumers in that it wants
different function and field names from the exposed attribute name, so
it keeps it's local macros to define attributes instead of using the common
ones.

Signed-off-by: Christoph Hellwig <hch@lst.de>
Reviewed-by: Andrzej Pietrasiewicz <andrzej.p@samsung.com>
Acked-by: Felipe Balbi <balbi@ti.com>
Signed-off-by: Nicholas Bellinger <nab@linux-iscsi.org>