usb: gadget: uvc: Replace plain printk() with dev_*()

Adding device context to the kernel log messages make them more useful.
Add new uvcg_* macros based on dev_*() that print both the gadget device
name and the function name.

While at it, remove a commented out printk statement and an unused
printk-based macro.

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>
Reviewed-by: Kieran Bingham <kieran.bingham@ideasonboard.com>