media: uvcvideo: Fix dereference of out-of-bound list iterator

Fixes the following coccinelle report:

drivers/media/usb/uvc/uvc_ctrl.c:1860:5-11:
ERROR: invalid reference to the index variable of the iterator on line 1854

by adding a boolean variable to check if the loop has found the

Found using - Coccinelle (http://coccinelle.lip6.fr

)

[Replace cursor variable with bool found]

Signed-off-by: Daniel W. S. Almeida <dwlsalmeida@gmail.com>
Signed-off-by: Laurent Pinchart <laurent.pinchart@ideasonboard.com>
Signed-off-by: Mauro Carvalho Chehab <mchehab+huawei@kernel.org>