Instead of calling prink() inside uvc_trace, use dev_printk(), which adds
context to the output.

Now that we are at it, regroup the strings so the messages can be easily
"grepable".
Suggested-by: Laurent Pinchart <laurent.pinchart@ideasonboard.com>
Signed-off-by: Ricardo Ribalda <ribalda@chromium.org>
Signed-off-by: Laurent Pinchart <laurent.pinchart@ideasonboard.com>
Signed-off-by: Mauro Carvalho Chehab <mchehab+huawei@kernel.org>